Updates Yoga to handle being in a Xcode framework project

Summary:
Updates the Yoga Podspec to work with Cocoa frameworks. Replicated a2's work over on Yoga facebook/yoga#726

(Which should also get merged too please ❤️ )

Set it manually in my own project and tested 👍

Only facebook/yoga#726

[IOS] [BUGFIX] [Yoga] - Fixes to yoga.podspec when integrating into an iOS app using Cocoa frameworks.
Closes #18492

Differential Revision: D7355907

Pulled By: hramos

fbshipit-source-id: 6986d2bc560a23bb7f0f20b80e88bb440079891a

ReactCommon/yoga/yoga.podspec

@@ -36,9 +36,13 @@ Pod::Spec.new do |spec|
   # Pinning to the same version as React.podspec.
   spec.platforms = { :ios => "8.0", :tvos => "9.2" }
 
-  # Set this environment variable when not using the `:path` option to install the pod.
+  # Set this environment variable when *not* using the `:path` option to install the pod.
   # E.g. when publishing this spec to a spec repo.
   source_files = 'yoga/**/*.{cpp,h}'
   source_files = File.join('ReactCommon/yoga', source_files) if ENV['INSTALL_YOGA_WITHOUT_PATH_OPTION']
   spec.source_files = source_files
+
+  header_files = 'yoga/{Yoga,YGEnums,YGMacros}.h'
+  header_files = File.join('ReactCommon/yoga', header_files) if ENV['INSTALL_YOGA_WITHOUT_PATH_OPTION']
+  spec.public_header_files = header_files
 end