Update React.podspec for RCTText

Summary:
Some of the classes of RCTText are now in the subfolders, this will fix pod integration for texts

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I have integrated React Native into my project with [this guide](https://facebook.github.io/react-native/docs/integration-with-existing-apps.html) and got the error `No component found for view with name RCTText`. Searching in the web did not give any useful information so I started to dig into the code and found out that there is now subfolders and pod taking only root classes. After making this change project started to compile again.

Create empty project with pods integrated add `RCTText` subspec and in `.js` file add  some `<Text>` element

Fixed `RCTText` subspec integration.
Closes #17655

Differential Revision: D6751039

Pulled By: hramos

fbshipit-source-id: f4538d1125af2b45f36e2fa535382e032dbc8f4e

Author: hovox

React.podspec

@@ -204,7 +204,7 @@ Pod::Spec.new do |s|
 
   s.subspec "RCTText" do |ss|
     ss.dependency             "React/Core"
-    ss.source_files         = "Libraries/Text/*.{h,m}"
+    ss.source_files         = "Libraries/Text/**/*.{h,m}"
   end
 
   s.subspec "RCTVibration" do |ss|