x11: Fix unused variable warnings

x11: Fix unused variable warnings

These are emitted when XInput 2 is not available.

```
../subprojects/gst-plugins-base/sys/ximage/ximagesink.c: In function ‘gst_x_image_sink_handle_xevents’:
../subprojects/gst-plugins-base/sys/ximage/ximagesink.c:696:29: warning: unused variable ‘state’ [-Wunused-variable]
  696 |   GstNavigationModifierType state = GST_NAVIGATION_MODIFIER_NONE;
      |                             ^~~~~
../subprojects/gst-plugins-base/sys/ximage/ximagesink.c:694:35: warning: unused variable ‘touch_frame_open’ [-Wunused-variable]
  694 |   gboolean pointer_moved = FALSE, touch_frame_open = FALSE;
      |                                   ^~~~~~~~~~~~~~~~
../subprojects/gst-plugins-base/sys/xvimage/xvimagesink.c: In function ‘gst_xv_image_sink_handle_xevents’:
../subprojects/gst-plugins-base/sys/xvimage/xvimagesink.c:427:35: warning: unused variable ‘touch_frame_open’ [-Wunused-variable]
  427 |   gboolean pointer_moved = FALSE, touch_frame_open = FALSE;
      |                                   ^~~~~~~~~~~~~~~~
../subprojects/gst-plugins-base/sys/xvimage/xvimagesink.c:426:29: warning: unused variable ‘state’ [-Wunused-variable]
  426 |   GstNavigationModifierType state = GST_NAVIGATION_MODIFIER_NONE;
      |                             ^~~~~
```

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/2093>