add missing import

after commit 92acee142065167bab1c71ff77ef9ded73f8fc8e
builds start failing with:
```
Undefined mixin 'background-image'.
  (in /builddir/build/BUILD/katello-git-348.990a3bf/app/assets/stylesheets/widgets/_org_switcher.scss)
/builddir/build/BUILD/katello-git-348.990a3bf/app/assets/stylesheets/widgets/_org_switcher.scss:61:in `background-image'
/builddir/build/BUILD/katello-git-348.990a3bf/app/assets/stylesheets/widgets/_org_switcher.scss:61
/opt/rh/ruby193/root/usr/share/gems/gems/sass-3.1.20/lib/sass/tree/visitors/perform.rb:170:in `visit_mixin'
/opt/rh/ruby193/root/usr/share/gems/gems/sass-3.1.20/lib/sass/tree/visitors/base.rb:37:in `visit'
/opt/rh/ruby193/root/usr/share/gems/gems/sass-3.1.20/lib/sass/tree/visitors/perform.rb:18:in `visit'
/opt/rh/ruby193/root/usr/share/gems/gems/sass-3.1.20/lib/sass/tree/visitors/base.rb:53:in `block in visit_children'
/opt/rh/ruby193/root/usr/share/gems/gems/sass-3.1.20/lib/sass/tree/visitors/base.rb:53:in `map'
/opt/rh/ruby193/root/usr/share/gems/gems/sass-3.1.20/lib/sass/tree/visitors/base.rb:53:in `visit_children'
/opt/rh/ruby193/root/usr/share/gems/gems/sass-3.1.20/lib/sass/tree/visitors/perform.rb:27:in `block in visit_children'
/opt/rh/ruby193/root/usr/share/gems/gems/sass-3.1.20/lib/sass/tree/visitors/perform.rb:39:in `with_environment'
/opt/rh/ruby193/root/usr/share/gems/gems/sass-3.1.20/lib/sass/tree/visitors/perform.rb:26:in `visit_children'
/opt/rh/ruby193/root/usr/share/gems/gems/sass-3.1.20/lib/sass/tree/visitors/base.rb:37:in `block in visit'
/opt/rh/ruby193/root/usr/share/gems/gems/sass-3.1.20/lib/sass/tree/visitors/perform.rb:238:in `visit_rule'
/opt/rh/ruby193/root/usr/share/gems/gems/sass-3.1.20/lib/sass/tree/visitors/base.rb:37:in `visit'
/opt/rh/ruby193/root/usr/share/gems/gems/sass-3.1.20/lib/sass/tree/visitors/perform.rb:18:in `visit'
/opt/rh/ruby193/root/usr/share/gems/gems/sass-3.1.20/lib/sass/tree/visitors/base.rb:53:in `block in visit_children'
/opt/rh/ruby193/root/usr/share/gems/gems/sass-3.1.20/lib/sass/tree/visitors/base.rb:53:in `map'
/opt/rh/ruby193/root/usr/share/gems/gems/sass-3.1.20/lib/sass/tree/visitors/base.rb:53:in `visit_children'
/opt/rh/ruby193/root/usr/share/gems/gems/sass-3.1.20/lib/sass/tree/visitors/perform.rb:27:in `block in visit_children'
/opt/rh/ruby193/root/usr/share/gems/gems/sass-3.1.20/lib/sass/tree/visitors/perform.rb:39:in `with_environment'
/opt/rh/ruby193/root/usr/share/gems/gems/sass-3.1.20/lib/sass/tree/visitors/perform.rb:26:in `visit_children'
/opt/rh/ruby193/root/usr/share/gems/gems/sass-3.1.20/lib/sass/tree/visitors/base.rb:37:in `block in visit'
/opt/rh/ruby193/root/usr/share/gems/gems/sass-3.1.20/lib/sass/tree/visitors/perform.rb:238:in `visit_rule'
/opt/rh/ruby193/root/usr/share/gems/gems/sass-3.1.20/lib/sass/tree/visitors/base.rb:37:in `visit'
/opt/rh/ruby193/root/usr/share/gems/gems/sass-3.1.20/lib/sass/tree/visitors/perform.rb:18:in `visit'
/opt/rh/ruby193/root/usr/share/gems/gems/sass-3.1.20/lib/sass/tree/visitors/base.rb:53:in `block in visit_children'
/opt/rh/ruby193/root/usr/share/gems/gems/sass-3.1.20/lib/sass/tree/visitors/base.rb:53:in `map'
/opt/rh/ruby193/root/usr/share/gems/gems/sass-3.1.20/lib/sass/tree/visitors/base.rb:53:in `visit_children'
/opt/rh/ruby193/root/usr/share/gems/gems/sass-3.1.20/lib/sass/tree/visitors/perform.rb:27:in `block in visit_children'
/opt/rh/ruby193/root/usr/share/gems/gems/sass-3.1.20/lib/sass/tree/visitors/perform.rb:39:in `with_environment'
/opt/rh/ruby193/root/usr/share/gems/gems/sass-3.1.20/lib/sass/tree/visitors/perform.rb:26:in `visit_children'
/opt/rh/ruby193/root/usr/share/gems/gems/sass-3.1.20/lib/sass/tree/visitors/base.rb:37:in `block in visit'
/opt/rh/ruby193/root/usr/share/gems/gems/sass-3.1.20/lib/sass/tree/visitors/perform.rb:238:in `visit_rule'
/opt/rh/ruby193/root/usr/share/gems/gems/sass-3.1.20/lib/sass/tree/visitors/base.rb:37:in `visit'
/opt/rh/ruby193/root/usr/share/gems/gems/sass-3.1.20/lib/sass/tree/visitors/perform.rb:18:in `visit'
/opt/rh/ruby193/root/usr/share/gems/gems/sass-3.1.20/lib/sass/tree/visitors/base.rb:53:in `block in visit_children'
/opt/rh/ruby193/root/usr/share/gems/gems/sass-3.1.20/lib/sass/tree/visitors/base.rb:53:in `map'
/opt/rh/ruby193/root/usr/share/gems/gems/sass-3.1.20/lib/sass/tree/visitors/base.rb:53:in `visit_children'
/opt/rh/ruby193/root/usr/share/gems/gems/sass-3.1.20/lib/sass/tree/visitors/perform.rb:27:in `block in visit_children'
/opt/rh/ruby193/root/usr/share/gems/gems/sass-3.1.20/lib/sass/tree/visitors/perform.rb:39:in `with_environment'
/opt/rh/ruby193/root/usr/share/gems/gems/sass-3.1.20/lib/sass/tree/visitors/perform.rb:26:in `visit_children'
/opt/rh/ruby193/root/usr/share/gems/gems/sass-3.1.20/lib/sass/tree/visitors/base.rb:37:in `block in visit'
/opt/rh/ruby193/root/usr/share/gems/gems/sass-3.1.20/lib/sass/tree/visitors/perform.rb:47:in `visit_root'
/opt/rh/ruby193/root/usr/share/gems/gems/sass-3.1.20/lib/sass/tree/visitors/base.rb:37:in `visit'
/opt/rh/ruby193/root/usr/share/gems/gems/sass-3.1.20/lib/sass/tree/visitors/perform.rb:18:in `visit'
/opt/rh/ruby193/root/usr/share/gems/gems/sass-3.1.20/lib/sass/tree/visitors/perform.rb:7:in `visit'
/opt/rh/ruby193/root/usr/share/gems/gems/sass-3.1.20/lib/sass/tree/root_node.rb:20:in `render'
/opt/rh/ruby193/root/usr/share/gems/gems/sass-3.1.20/lib/sass/engine.rb:300:in `_render'
/opt/rh/ruby193/root/usr/share/gems/gems/sass-3.1.20/lib/sass/engine.rb:247:in `render'
/opt/rh/ruby193/root/usr/share/gems/gems/sass-rails-3.2.5/lib/sass/rails/template_handlers.rb:106:in `evaluate'
/opt/rh/ruby193/root/usr/share/gems/gems/tilt-1.3.3/lib/tilt/template.rb:76:in `render'
/opt/rh/ruby193/root/usr/share/gems/gems/sprockets-2.4.5/lib/sprockets/context.rb:197:in `block in evaluate'
/opt/rh/ruby193/root/usr/share/gems/gems/sprockets-2.4.5/lib/sprockets/context.rb:194:in `each'
/opt/rh/ruby193/root/usr/share/gems/gems/sprockets-2.4.5/lib/sprockets/context.rb:194:in `evaluate'
/opt/rh/ruby193/root/usr/share/gems/gems/sprockets-2.4.5/lib/sprockets/processed_asset.rb:12:in `initialize'
/opt/rh/ruby193/root/usr/share/gems/gems/sprockets-2.4.5/lib/sprockets/base.rb:350:in `new'
/opt/rh/ruby193/root/usr/share/gems/gems/sprockets-2.4.5/lib/sprockets/base.rb:350:in `block in build_asset'
/opt/rh/ruby193/root/usr/share/gems/gems/sprockets-2.4.5/lib/sprockets/base.rb:371:in `circular_call_protection'
/opt/rh/ruby193/root/usr/share/gems/gems/sprockets-2.4.5/lib/sprockets/base.rb:349:in `build_asset'
/opt/rh/ruby193/root/usr/share/gems/gems/sprockets-2.4.5/lib/sprockets/index.rb:93:in `block in build_asset'
/opt/rh/ruby193/root/usr/share/gems/gems/sprockets-2.4.5/lib/sprockets/caching.rb:58:in `cache_asset'
/opt/rh/ruby193/root/usr/share/gems/gems/sprockets-2.4.5/lib/sprockets/index.rb:92:in `build_asset'
/opt/rh/ruby193/root/usr/share/gems/gems/sprockets-2.4.5/lib/sprockets/base.rb:270:in `find_asset'
/opt/rh/ruby193/root/usr/share/gems/gems/sprockets-2.4.5/lib/sprockets/index.rb:60:in `find_asset'
/opt/rh/ruby193/root/usr/share/gems/gems/sprockets-2.4.5/lib/sprockets/bundled_asset.rb:16:in `initialize'
/opt/rh/ruby193/root/usr/share/gems/gems/sprockets-2.4.5/lib/sprockets/base.rb:353:in `new'
/opt/rh/ruby193/root/usr/share/gems/gems/sprockets-2.4.5/lib/sprockets/base.rb:353:in `build_asset'
/opt/rh/ruby193/root/usr/share/gems/gems/sprockets-2.4.5/lib/sprockets/index.rb:93:in `block in build_asset'
/opt/rh/ruby193/root/usr/share/gems/gems/sprockets-2.4.5/lib/sprockets/caching.rb:58:in `cache_asset'
/opt/rh/ruby193/root/usr/share/gems/gems/sprockets-2.4.5/lib/sprockets/index.rb:92:in `build_asset'
/opt/rh/ruby193/root/usr/share/gems/gems/sprockets-2.4.5/lib/sprockets/base.rb:270:in `find_asset'
/opt/rh/ruby193/root/usr/share/gems/gems/sprockets-2.4.5/lib/sprockets/index.rb:60:in `find_asset'
/opt/rh/ruby193/root/usr/share/gems/gems/sprockets-2.4.5/lib/sprockets/environment.rb:74:in `find_asset'
/opt/rh/ruby193/root/usr/share/gems/gems/actionpack-3.2.8/lib/sprockets/static_compiler.rb:23:in `block in compile'
/opt/rh/ruby193/root/usr/share/gems/gems/sprockets-2.4.5/lib/sprockets/base.rb:320:in `block in each_logical_path'
/opt/rh/ruby193/root/usr/share/gems/gems/sprockets-2.4.5/lib/sprockets/base.rb:307:in `block (2 levels) in each_file'
/opt/rh/ruby193/root/usr/share/gems/gems/sprockets-2.4.5/lib/sprockets/base.rb:297:in `each'
/opt/rh/ruby193/root/usr/share/gems/gems/sprockets-2.4.5/lib/sprockets/base.rb:297:in `each_entry'
/opt/rh/ruby193/root/usr/share/gems/gems/sprockets-2.4.5/lib/sprockets/base.rb:305:in `block in each_file'
/opt/rh/ruby193/root/usr/share/gems/gems/sprockets-2.4.5/lib/sprockets/base.rb:304:in `each'
/opt/rh/ruby193/root/usr/share/gems/gems/sprockets-2.4.5/lib/sprockets/base.rb:304:in `each_file'
/opt/rh/ruby193/root/usr/share/gems/gems/sprockets-2.4.5/lib/sprockets/base.rb:318:in `each_logical_path'
/opt/rh/ruby193/root/usr/share/gems/gems/actionpack-3.2.8/lib/sprockets/static_compiler.rb:18:in `compile'
/opt/rh/ruby193/root/usr/share/gems/gems/actionpack-3.2.8/lib/sprockets/assets.rake:56:in `internal_precompile'
/opt/rh/ruby193/root/usr/share/gems/gems/actionpack-3.2.8/lib/sprockets/assets.rake:74:in `block (3 levels) in <top (required)>'
/opt/rh/ruby193/root/usr/share/ruby/rake/task.rb:205:in `call'
/opt/rh/ruby193/root/usr/share/ruby/rake/task.rb:205:in `block in execute'
/opt/rh/ruby193/root/usr/share/ruby/rake/task.rb:200:in `each'
/opt/rh/ruby193/root/usr/share/ruby/rake/task.rb:200:in `execute'
/opt/rh/ruby193/root/usr/share/ruby/rake/task.rb:158:in `block in invoke_with_call_chain'
/opt/rh/ruby193/root/usr/share/ruby/monitor.rb:211:in `mon_synchronize'
/opt/rh/ruby193/root/usr/share/ruby/rake/task.rb:151:in `invoke_with_call_chain'
/opt/rh/ruby193/root/usr/share/ruby/rake/task.rb:144:in `invoke'
/opt/rh/ruby193/root/usr/share/ruby/rake/application.rb:116:in `invoke_task'
/opt/rh/ruby193/root/usr/share/ruby/rake/application.rb:94:in `block (2 levels) in top_level'
/opt/rh/ruby193/root/usr/share/ruby/rake/application.rb:94:in `each'
/opt/rh/ruby193/root/usr/share/ruby/rake/application.rb:94:in `block in top_level'
/opt/rh/ruby193/root/usr/share/ruby/rake/application.rb:133:in `standard_exception_handling'
/opt/rh/ruby193/root/usr/share/ruby/rake/application.rb:88:in `top_level'
/opt/rh/ruby193/root/usr/share/ruby/rake/application.rb:66:in `block in run'
/opt/rh/ruby193/root/usr/share/ruby/rake/application.rb:133:in `standard_exception_handling'
/opt/rh/ruby193/root/usr/share/ruby/rake/application.rb:63:in `run'
/opt/rh/ruby193/root/usr/bin/rake:32:in `<main>'
```