licensing: add missing header to one .network file

It should have the full header because it will be installed onto
user systems like the other .network files.

network/80-wifi-adhoc.network

@@ -1,3 +1,12 @@
+#  SPDX-License-Identifier: LGPL-2.1-or-later
+#
+#  This file is part of systemd.
+#
+#  systemd is free software; you can redistribute it and/or modify it
+#  under the terms of the GNU Lesser General Public License as published by
+#  the Free Software Foundation; either version 2.1 of the License, or
+#  (at your option) any later version.
+
 [Match]
 Type=wlan
 WLANInterfaceType=ad-hoc