exclude React/Fabric/ from core

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React native does not build on iOS when using Podspec environment. I tried the solution proposed in facebook#18683 and got it to build.

Fixes facebook#18683

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[IOS] [BUGFIX] [React.podspec] - Exclude `React/Fabric/*` from Core subspec
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    CATEGORY
  [----------]      TYPE
  [ CLI      ] [-------------]    LOCATION
  [ DOCS     ] [ BREAKING    ] [-------------]
  [ GENERAL  ] [ BUGFIX      ] [ {Component} ]
  [ INTERNAL ] [ ENHANCEMENT ] [ {Filename}  ]
  [ IOS      ] [ FEATURE     ] [ {Directory} ]   |-----------|
  [ ANDROID  ] [ MINOR       ] [ {Framework} ] - | {Message} |
  [----------] [-------------] [-------------]   |-----------|

 EXAMPLES:

 [IOS] [BREAKING] [FlatList] - Change a thing that breaks other things
 [ANDROID] [BUGFIX] [TextInput] - Did a thing to TextInput
 [CLI] [FEATURE] [local-cli/info/info.js] - CLI easier to do things with
 [DOCS] [BUGFIX] [GettingStarted.md] - Accidentally a thing/word
 [GENERAL] [ENHANCEMENT] [Yoga] - Added new yoga thing/position
 [INTERNAL] [FEATURE] [./scripts] - Added thing to script that nobody will see
-->
Closes facebook#18688

Differential Revision: D7505267

Pulled By: hramos

fbshipit-source-id: 8e7138ee966d08209a74193e418feaecceefb86a

React.podspec

@@ -51,6 +51,7 @@ Pod::Spec.new do |s|
                               "React/Inspector/*",
                               "ReactCommon/yoga/*",
                               "React/Cxx*/*",
+                              "React/Fabric/*",
     ss.ios.exclude_files    = "React/**/RCTTV*.*"
     ss.tvos.exclude_files   = "React/Modules/RCTClipboard*",
                               "React/Views/RCTDatePicker*",