Fix "undefined local variable or method `stdscr'" error. This is happeni...

I'm using the knife-essentials gem for chef on OSX, which uses highline. This error doesn't seem to be something that's dependent on that context, though.

Here's the stack trace I got:

```
{gems_root}/highline-1.6.19/lib/highline/system_extensions.rb:179:in `terminal_size': undefined local variable or method `stdscr' for HighLine::SystemExtensions:Module (NameError)
	from {gems_root}/knife-essentials-1.5.4/lib/chef/knife/list_essentials.rb:128:in `print_results'
	from {gems_root}/knife-essentials-1.5.4/lib/chef/knife/list_essentials.rb:118:in `print_result_paths'
	from {gems_root}/knife-essentials-1.5.4/lib/chef/knife/list_essentials.rb:86:in `run'
	from {gems_root}/chef-11.6.0/lib/chef/knife.rb:466:in `run_with_pretty_exceptions'
	from {gems_root}/chef-11.6.0/lib/chef/knife.rb:173:in `run'
	from {gems_root}/chef-11.6.0/lib/chef/application/knife.rb:123:in `run'
	from {gems_root}/chef-11.6.0/bin/knife:25:in `<top (required)>'
```

Adding in the namespace of FFI::NCurses seems to fix the problem.