Properly generating well known proto files for the macos build.

ruby/Rakefile

@@ -69,6 +69,7 @@ else
 
   if RUBY_PLATFORM =~ /darwin/
     task 'gem:native' do
+      system "rake genproto"
       system "rake cross native gem RUBY_CC_VERSION=2.3.0:2.2.2:2.1.5:2.0.0"
     end
   else