shell: Use the correct the member name when iterating panel_list

It does not get the correct panel height for now.  The memeber of list
to read should be layer_link not link now.

Signed-off-by: Juan Zhao <juan.j.zhao@linux.intel.com>

src/shell.c

@@ -1331,7 +1331,7 @@ get_output_panel_height(struct desktop_shell *shell,
 	if (!output)
 		return 0;
 
-	wl_list_for_each(surface, &shell->panel_layer.surface_list, link) {
+	wl_list_for_each(surface, &shell->panel_layer.surface_list, layer_link) {
 		if (surface->output == output) {
 			panel_height = surface->geometry.height;
 			break;