Add folly_compiler_flags to Core subspec (facebook#23488)

Summary:
This fixes the following iOS build error when using React via pods (using the new Xcode build system);

> project/Pods/Folly/folly/portability/Config.h:20:10: 'folly/folly-config.h' file not found

react-native-community/releases#79 (comment)
react-native-community/releases#79 (comment)
react-native-community/releases#79 (comment)

[iOS] [Fixed] - Fixed a "'folly/folly-config.h' file not found" build error when using React via CocoaPods
Pull Request resolved: facebook#23488

Differential Revision: D14121296

Pulled By: cpojer

fbshipit-source-id: 5eb2ba3066aef2bc9cfb95a9172cbef921c47170

Author: Salakar

React.podspec

@@ -66,6 +66,7 @@ Pod::Spec.new do |s|
                               "React/Views/RCTSlider*",
                               "React/Views/RCTSwitch*",
                               "React/Views/RCTWebView*"
+    ss.compiler_flags       = folly_compiler_flags
     ss.header_dir           = "React"
     ss.framework            = "JavaScriptCore"
     ss.libraries            = "stdc++"