Don't try to call IOS_SupportedHIDDevice() if it's not defined

src/joystick/darwin/SDL_iokitjoystick.c

@@ -526,11 +526,15 @@ GetDeviceInfo(IOHIDDeviceRef hidDevice, recDevice *pDevice)
 static SDL_bool
 JoystickAlreadyKnown(IOHIDDeviceRef ioHIDDeviceObject)
 {
+    recDevice *i;
+
+#if TARGET_OS_OSX
     extern SDL_bool IOS_SupportedHIDDevice(IOHIDDeviceRef device);
     if (IOS_SupportedHIDDevice(ioHIDDeviceObject)) {
         return SDL_TRUE;
     }
-    recDevice *i;
+#endif
+
     for (i = gpDeviceList; i != NULL; i = i->pNext) {
         if (i->deviceRef == ioHIDDeviceObject) {
             return SDL_TRUE;

src/joystick/iphoneos/SDL_mfijoystick.m

@@ -1365,7 +1365,6 @@ IOS_JoystickGetGamepadMapping(int device_index, SDL_GamepadMapping *out)
 }
 
 #if TARGET_OS_OSX
-extern SDL_bool IOS_SupportedHIDDevice(IOHIDDeviceRef device);
 SDL_bool IOS_SupportedHIDDevice(IOHIDDeviceRef device)
 {
 #ifdef SDL_JOYSTICK_MFI