ubus_monitor: workaround possibly false positive uses of memory after it is freed

scan-build from clang-9 has reported following:

 libubox/list.h:141:2: warning: Use of memory after it is freed
        _list_add(_new, head, head->next);

Signed-off-by: Petr Štetiar <ynezz@true.cz>

ubusd_monitor.c

@@ -41,7 +41,7 @@ ubusd_monitor_connect(struct ubus_client *cl, struct ubus_msg_buf *ub)
 		return false;
 
 	m->cl = cl;
-	list_add(&m->list, &monitors);
+	list_add_tail(&m->list, &monitors);
 
 	return true;
 }