wayland: Fix c&p typo in wl_listener notify callback

The corresponding wl_notify field for destroy_data_device_icon()
is drag_grab->drag_icon_listener, otherwise we're fetching a pointer
that's slightly off where we want.

src/wayland/meta-wayland-data-device.c

@@ -371,7 +371,7 @@ static void
 destroy_data_device_icon (struct wl_listener *listener, void *data)
 {
   MetaWaylandDragGrab *drag_grab =
-    wl_container_of (listener, drag_grab, drag_data_source_listener);
+    wl_container_of (listener, drag_grab, drag_icon_listener);
 
   drag_grab->drag_surface = NULL;