SDL2: include cf-private on darwin

It's not included implicitly by the frameworks anymore.

    Undefined symbols for architecture x86_64:
      "_NSDefaultRunLoopMode", referenced from:
          _Cocoa_PumpEvents in SDL_cocoaevents.o
      "_NSURLIsAliasFileKey", referenced from:
          -[SDLWindow performDragOperation:] in SDL_cocoawindow.o
      "_OBJC_CLASS_$_NSArray", referenced from:
          objc-class-ref in SDL_cocoaclipboard.o
          objc-class-ref in SDL_cocoakeyboard.o
          objc-class-ref in SDL_cocoawindow.o
      "_OBJC_CLASS_$_NSData", referenced from:
          objc-class-ref in SDL_cocoamouse.o
      "_OBJC_CLASS_$_NSDate", referenced from:
          objc-class-ref in SDL_cocoaevents.o
      "_OBJC_CLASS_$_NSDictionary", referenced from:
          objc-class-ref in SDL_cocoaevents.o
      "_OBJC_CLASS_$_NSMutableArray", referenced from:
          objc-class-ref in SDL_cocoawindow.o
      "_OBJC_CLASS_$_NSURL", referenced from:
          objc-class-ref in SDL_cocoawindow.o
      "_OBJC_CLASS_$_NSUserDefaults", referenced from:
          objc-class-ref in SDL_cocoaevents.o
      "_OBJC_EHTYPE_$_NSException", referenced from:
          GCC_except_table67 in SDL_cocoawindow.o
    ld: symbol(s) not found for architecture x86_64