rake/backtrace.rb: remove wrong value

* lib/rake/backtrace.rb (Rake::Backtrace::SYS_KEYS): only names
  end with prefix or libdir, not just include them.  libdirname is
  not a key of path name.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47199 b2dd03c8-39d4-4d8f-98ff-823fe69b080e