compositor: Track pointer image protocol change

The request pointer.attach was replaced with pointer.set_cursor that
lets a client set the cursor surface for a given pointer.
diff --git a/src/compositor.h b/src/compositor.h
index b5136fc..4147811 100644
--- a/src/compositor.h
+++ b/src/compositor.h
@@ -207,6 +207,7 @@
 
 	struct weston_compositor *compositor;
 	struct weston_surface *sprite;
+	struct wl_listener sprite_destroy_listener;
 	struct weston_surface *drag_surface;
 	struct wl_listener drag_surface_destroy_listener;
 	int32_t hotspot_x, hotspot_y;