Fix missing copyright header

tokio-xmpp/src/event.rs

@@ -1,3 +1,9 @@
+// Copyright (c) 2024 Jonas Schäfer <jonas@zombofant.net>
+//
+// This Source Code Form is subject to the terms of the Mozilla Public
+// License, v. 2.0. If a copy of the MPL was not distributed with this
+// file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
 use rand::{thread_rng, Rng};
 use xmpp_parsers::{iq::Iq, jid::Jid, message::Message, presence::Presence};