use of undeclared identifier UIApplicationOpenSettingsURLString

I write Unity Plugin (Open Setting in device).

extern "C"
{
void _OpenSettings()
{
  [[UIApplication sharedApplication] openURL:[NSURL URLWithString:UIApplicationOpenSettingsURLString]];
}
}

But my XCode show error: "use of undeclared identifier UIApplicationOpenSettingsURLString" XCode version 5.1.1 (I can not update for some reason). Please help me. Thsnks!