[ReactNative] Fix extern on RCTProfile.h

Summary:
@public

Add missing `RCT_EXTERN` to the constants in `RCTProfile`'s header

Test Plan:
Move the the import on `RCTBridge` to the header, build no longer fails due to
duplicate symbols.

React/Base/RCTProfile.h

@@ -20,8 +20,8 @@
  * before before using it.
  */
 
-NSString *const RCTProfileDidStartProfiling;
-NSString *const RCTProfileDidEndProfiling;
+RCT_EXTERN NSString *const RCTProfileDidStartProfiling;
+RCT_EXTERN NSString *const RCTProfileDidEndProfiling;
 
 #if RCT_DEV