Add back in the g_unix_signal_add which appears to have been dropped

(cherry picked from commit 2a6974a)

src/lightdm-webkit2-greeter.c

@@ -215,6 +215,7 @@ main(int argc, char **argv) {
 	textdomain (GETTEXT_PACKAGE);
 
 	gtk_init(&argc, &argv);
+	g_unix_signal_add(SIGTERM, (GSourceFunc) quit_cb, /* is callback */ NULL);
 
 	/* Apply greeter settings from config file */
 	keyfile = g_key_file_new();