CMake: add SPDX identifiers to top-level CMakeLists.txt

- This is kind of bogus, since the file has existed for much longer
  than the identifier says, but I don't want to do spelunking now.

CMakeLists.txt

@@ -1,5 +1,8 @@
 # === This file is part of Calamares - <https://github.com/calamares> ===
 #
+#   SPDX-FileCopyrightText: 2017 Adriaan de Groot <groot@kde.org>
+#   SPDX-License-Identifier: GPL-3.0-or-later
+#
 #   Calamares is free software: you can redistribute it and/or modify
 #   it under the terms of the GNU General Public License as published by
 #   the Free Software Foundation, either version 3 of the License, or
@@ -13,9 +16,6 @@
 #   You should have received a copy of the GNU General Public License
 #   along with Calamares. If not, see <http://www.gnu.org/licenses/>.
 #
-#   SPDX-License-Identifier: GPL-3.0-or-later
-#   License-Filename: LICENSE
-#
 ###
 #
 # Generally, this CMakeLists.txt will find all the dependencies for Calamares