x11: Add #ifdef guards to the XInput2 quit function

Fixes building if XInput2 isn't available.

src/video/x11/SDL_x11xinput2.c

@@ -342,6 +342,7 @@ bool X11_InitXinput2(SDL_VideoDevice *_this)
 
 void X11_QuitXinput2(SDL_VideoDevice *_this)
 {
+#ifdef SDL_VIDEO_DRIVER_X11_XINPUT2
     SDL_free(xinput2_pointer_button_map);
     xinput2_pointer_button_map = NULL;
     xinput2_pointer_button_map_size = 0;
@@ -354,6 +355,7 @@ void X11_QuitXinput2(SDL_VideoDevice *_this)
     scrollable_devices = NULL;
     scrollable_device_count = 0;
 #endif
+#endif
 }
 
 void X11_Xinput2UpdatePointerMapping(SDL_VideoDevice *_this)