rename FBMacros.h to FBDefines.h

Summary:
Renaming FBMacros.h to FBDefines.h for more standard naming (similar to say UIKit/UIKit.h or Foundation/Foundation.h).

```
Tools/fastmod/fastmod --accept-all '\bFBMacros.h' FBDefines.h ../xplat
```

drop-conflicts
bypass-lint
allow-large-files

Reviewed By: adamjernst

Differential Revision: D14011897

fbshipit-source-id: f1dd7e686eef2cdb109e85c2db4a0f2ac3d0ba42

iOS/FBDefines/FBDefines.h

@@ -0,0 +1,13 @@
+/*
+ *  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)
+    #define FB_LINKABLE(NAME)
+    #define FB_LINK_REQUIRE_CATEGORY(NAME)
+ #endif