community/ruby-rdiscount: add missing patch file

community/ruby-rdiscount/fix-rakefile.patch

@@ -0,0 +1,12 @@
+Ruby 2.5+ doesn't support option -rubygems anymore.
+
+--- a/Rakefile
++++ b/Rakefile
+@@ -53,7 +53,6 @@
+ require 'rake/testtask'
+ Rake::TestTask.new('test:unit') do |t|
+   t.test_files = FileList['test/*_test.rb']
+-  t.ruby_opts += ['-rubygems'] if defined? Gem
+ end
+ task 'test:unit' => [:build]
+