Fix the missing declaration of the GKLeaderboard identifier on OS X 10.9.

GKLeaderboardIdentifier.m

#import <Foundation/Foundation.h>
#import <GameKit/GameKit.h>

@interface GKLeaderboard (IdentifierSupport)

@property (copy, readonly) NSString *identifier;

@end


int main(int argc, const char * argv[])
{
	@autoreleasepool
	{
		GKLeaderboard *leaderboard = [[GKLeaderboard alloc] init];
		
		leaderboard.category = @"1234";
		
		NSLog(@"category :%@", leaderboard.category);
		NSLog(@"identifier :%@", leaderboard.category);
	}
	return 0;
}