Add print.css to asset precompile array.

config/application.rb

@@ -64,6 +64,7 @@ module Gitlab
     config.assets.enabled = true
     config.assets.paths << Emoji.images_path
     config.assets.precompile << "emoji/*.png"
+    config.assets.precompile << "print.css"
 
     # Version of your assets, change this if you want to expire all your assets
     config.assets.version = '1.0'