Fix React-graphics podspec source_files (facebook#48216)

Summary:
Pull Request resolved: facebook#48216

The `React-graphics` podspec is manually excluding some paths from the source_files but this approach is error prone. For esample changes that added new paths that must be excluded can create failures.

This change the podspec file to explicitly add only the files that iOS requires.

## Changelog:
[iOS][Changed] - Explicitly define the source files for React-graphics

## Facebook:
This change is necessary because we added the macos platform that was not included before and now we can't build RNTester from fbsource using the internal pipeline.

Reviewed By: hoxyq

Differential Revision: D67095677

fbshipit-source-id: 701d5938f6e141a313be62c8f930a089e1d6ee96

packages/react-native/ReactCommon/react/renderer/graphics/React-graphics.podspec

@@ -24,7 +24,7 @@ boost_config = get_boost_config()
 boost_compiler_flags = boost_config[:compiler_flags]
 
 Pod::Spec.new do |s|
-  source_files = "**/*.{m,mm,cpp,h}"
+  source_files = "*.{m,mm,cpp,h}", "platform/ios/**/*.{m,mm,cpp,h}"
   header_search_paths = [
     "\"$(PODS_ROOT)/boost\"",
     "\"$(PODS_TARGET_SRCROOT)/../../../\"",
@@ -44,10 +44,6 @@ Pod::Spec.new do |s|
   s.source                 = source
   s.compiler_flags         = folly_compiler_flags + ' ' + boost_compiler_flags
   s.source_files           = source_files
-  s.exclude_files          = "tests",
-                             "platform/android",
-                             "platform/cxx",
-                             "platform/windows",
   s.header_dir             = "react/renderer/graphics"
   s.framework = "UIKit"