fix: Fix Xcode 12 compatibility (#138)

Reference: facebook/react-native#29633 (comment)

Author: fmorau

react-native-udp.podspec

@@ -12,6 +12,6 @@ Pod::Spec.new do |s|
   s.platform       = :ios, "7.0"
   s.source         = { :git => package_json["repository"]["url"].gsub(/(http.*)/).first, :tag => "v#{s.version}" }
   s.source_files   = 'ios/**/*.{h,m}'
-  s.dependency 'React'
+  s.dependency 'React-Core'
   s.dependency 'CocoaAsyncSocket'
 end