xorg/50-systemd-user: add a full license header

This file is installed onto user systems, so it should have the full
header that says where it came from.

xorg/50-systemd-user.sh

@@ -1,4 +1,12 @@
 #!/bin/sh
+#  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.
 
 systemctl --user import-environment DISPLAY XAUTHORITY