Fix missing require in x11_dependency.rb

Fixes Homebrew/homebrew#18918.

Fix missing require in x11_dependency.rb
d13a39d1 · Jack Nagel · 4ebafbf6 · d13a39d1
Commit d13a39d1 authored 12 years ago by Jack Nagel
--- a/Library/Homebrew/requirements/x11_dependency.rb
+++ b/Library/Homebrew/requirements/x11_dependency.rb
 require 'requirement'
+require 'extend/set'

 class X11Dependency < Requirement
  include Comparable