add a dependency on the UnityFramework

Author: krispypen

ios/flutter_unity_widget.podspec

@@ -15,6 +15,7 @@ Flutter unity 3D widget for embedding unity in flutter
   s.source_files = 'Classes/**/*'
   s.public_header_files = 'Classes/**/*.h'
   s.dependency 'Flutter'
+  s.frameworks = 'UnityFramework'
 
   s.ios.deployment_target = '8.0'
   s.xcconfig = {