Fix Obj-C and Swift builds which are broken in v0.6.13 (#192)

Summary:
9f807c introduced a new subspec in SonarKit.podspec, which caused Cpp header files to be included in `SonarKit-umbrella.h`, causing all non-ObjC++ project integrations to fail to build (see #190 for more info). SirArkimedes and I investigated this today and found the fix:

1. Make the `CppBridge` subspec's header files private
2. Add a hack to make Cocoapods copy `FBMacros.h` into `{PODS_ROOT}/Headers/Public/SonarKit`, since otherwise `#import "FBMacros.h"` in `SonarKit-umbrella.h` will fail to resolve in out-of-the-box projects.

I've put up a branch [here](https://github.com/noahsark769/sonar-testing-example-projects/tree/working_reference) which pulls the pod from this branch and verifies that the fix works.

Fixes #190
Pull Request resolved: https://github.com/facebook/Sonar/pull/192

Reviewed By: passy

Differential Revision: D9049420

Pulled By: priteshrnandgaonkar

fbshipit-source-id: 9383016975607e897085323d64ad44a068199918

iOS/FBDefines/FBMacros.h

@@ -1,16 +0,0 @@
-/*
- *  Copyright (c) 2004-present, Facebook, Inc.
- *
- *  This source code is licensed under the MIT license found in the LICENSE
- *  file in the root directory of this source tree.
- *
- */
-#ifndef FB_SK_MACROS_H
- #define FB_SK_MACROS_H
-
-    #define FB_LINK_REQUIRE_(NAME, UNIQUE)
-    #define FB_LINKABLE(NAME)
-    #define FB_LINK_REQUIRE(NAME)
-    #define FB_LINK_REQUIRE_EXT(NAME)
-
-#endif