[xcassets] fix: add resources to Copy Resources Phases, not as Compile Sources Phase

lib/cocoapods/installer/user_project_integrator/target_integrator.rb

@@ -152,7 +152,7 @@ module Pod
 
               UI.puts "   - Adding #{resource_path.basename} to targets #{native_targets.map(&:name)}"
               native_targets.each do |user_target|
-                user_target.add_file_references([file_ref])
+                user_target.add_resources([file_ref])
               end              
             end
           end