backends/native: Pass the axes down to the relative clutter motion event

Otherwise we get a happy segfault when we're trying broadcast_axis()
during the tablet event handling.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/3410>

src/backends/native/meta-virtual-input-device-native.c

@@ -226,7 +226,8 @@ notify_relative_motion_in_impl (GTask *task)
 						 virtual_evdev->impl_state->device,
 						 event->time_us,
 						 event->x, event->y,
-						 event->x, event->y);
+						 event->x, event->y,
+						 NULL);
   g_task_return_boolean (task, TRUE);
   return G_SOURCE_REMOVE;
 }