fix invalid pointer warning at build time/

src/lightdm-webkit2-greeter.c

@@ -190,7 +190,7 @@ main(int argc, char **argv) {
 	textdomain (GETTEXT_PACKAGE);
 
 	gtk_init(&argc, &argv);
-	g_unix_signal_add(SIGTERM, gtk_main_quit, NULL);
+	g_unix_signal_add(SIGTERM, (GSourceFunc) gtk_main_quit, NULL);
 
 	/* Apply greeter settings from config file */
 	keyfile = g_key_file_new();