xwayland: emit set_parent signal when parent is destroyed

When a xwayland surface is destroyed, its children's parent pointer
is set to NULL, but the set_parent signal was not emitted.

(cherry picked from commit a94cd29eb1)

xwayland/xwm.c

@@ -630,6 +630,7 @@ static void xwayland_surface_destroy(struct wlr_xwayland_surface *xsurface) {
 		wl_list_remove(&child->parent_link);
 		wl_list_init(&child->parent_link);
 		child->parent = NULL;
+		wl_signal_emit_mutable(&child->events.set_parent, NULL);
 	}
 
 	wl_list_remove(&xsurface->unpaired_link);