SDL: include cf-private on darwin

It's not included implicitly by the frameworks anymore.

    Undefined symbols for architecture x86_64:
      "_NSDefaultRunLoopMode", referenced from:
          _QZ_PumpEvents in SDL_QuartzEvents.o
      "_OBJC_CLASS_$_NSArray", referenced from:
          objc-class-ref in SDL_QuartzEvents.o
      "_OBJC_CLASS_$_NSDate", referenced from:
          objc-class-ref in SDL_QuartzEvents.o
    ld: symbol(s) not found for architecture x86_64