fix: add React-Core to podspec for Xcode 12 compatibility

https://github.com/facebook/react-native/issues/29633

react-native-webview.podspec

@@ -16,4 +16,5 @@ Pod::Spec.new do |s|
   s.source_files  = "apple/**/*.{h,m}"
 
   s.dependency 'React'
+  s.dependency 'React-Core'
 end