Mark global variable as extern "C"

Reviewed By: mnovakovic

Differential Revision: D7369214

fbshipit-source-id: 710d817b4bf74b5ca0621645dd5c005b778d7171

React/Base/RCTRootView.h

@@ -30,7 +30,12 @@ typedef NS_ENUM(NSInteger, RCTRootViewSizeFlexibility) {
  * after the application has loaded. This is used to hide the `loadingView`, and
  * is a good indicator that the application is ready to use.
  */
-extern NSString *const RCTContentDidAppearNotification;
+#if defined(__cplusplus)
+extern "C"
+#else
+extern
+#endif
+NSString *const RCTContentDidAppearNotification;
 
 /**
  * Native view used to host React-managed views within the app. Can be used just