Add license headers and SPDX identifiers to meson.build files

So far I avoided adding license headers to meson files, but they are pretty
big and important and should carry license headers like everything else.
I added my own copyright, even though other people modified those files too.
But this is mostly symbolic, so I hope that's OK.

sysusers.d/meson.build

@@ -1,3 +1,20 @@
+# SPDX-License-Identifier: LGPL-2.1+
+#
+# Copyright 2017 Zbigniew Jędrzejewski-Szmek
+#
+# 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.
+#
+# systemd is distributed in the hope that it will be useful, but
+# WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
+# Lesser General Public License for more details.
+#
+# You should have received a copy of the GNU Lesser General Public License
+# along with systemd; If not, see <http://www.gnu.org/licenses/>.
+
 in_files = ['basic.conf']
 
 enable_sysusers = conf.get('ENABLE_SYSUSERS') == 1