Fixing Yoga imports and includes on case-sensitive machines (#26416)

Summary:
In addition of the issue resolved by #26360 (already merged), machines with case-sensitive disks are still not able to build a project on Xcode due not found `<yoga/...` imports:

```
'yoga/Yoga.h' file not found
```

![Screen Shot 2019-09-12 at 11 15 54](https://user-images.githubusercontent.com/1728387/64791885-c58c9180-d54e-11e9-95cb-40befaab7acc.png)

## Changelog

[iOS] [Fixed] - Fix Yoga imports and includes
Pull Request resolved: #26416

Test Plan: `Build` command on Xcode now runs successfully in a Mac with disk in case-sensitive mode

Differential Revision: D17370392

Pulled By: PeteTheHeat

fbshipit-source-id: 2a225f47046113267adb154a4c6a9ef4664a12c3

ReactCommon/yoga/Yoga.podspec

@@ -28,6 +28,7 @@ Pod::Spec.new do |spec|
   spec.source = source
 
   spec.module_name = 'yoga'
+  spec.header_dir = 'yoga'
   spec.requires_arc = false
   spec.compiler_flags = [
       '-fno-omit-frame-pointer',