ios: exposed 'RCTConvert UITabBarSystemItem:' in the header file

Summary: This method was defined in the .m file already, so we can just expose it to the header for other usages.

Reviewed By: mmmulani, shergin

Differential Revision: D4557942

fbshipit-source-id: a1be8af8e31b88a152cabfab6d4cf82316088572

React/Views/RCTTabBarItem.h

@@ -10,6 +10,13 @@
 #import <UIKit/UIKit.h>
 
 #import <React/RCTComponent.h>
+#import <React/RCTConvert.h>
+
+@interface RCTConvert (UITabBarSystemItem)
+
++ (UITabBarSystemItem)UITabBarSystemItem:(id)json;
+
+@end
 
 @interface RCTTabBarItem : UIView