codegen: Add license and copyright headers to remaining files

The `.flake8` file has a trivial version history, so the copyright is
straightforward from that.

`meson.build` has a more complex history, but the only significant
contributions were from Centricular. From `git log
gio/gdbus-2.0/codegen/meson.build`, the other (following) commits are
too trivial to be copyrightable:
 - d10be6102f
 - 30b25a6fd9
 - 95fa229f34
 - 631c3534b7
 - 00d7568e4f
 - 9734e4854e
 - 65be80c3ed
 - 66e4ba806a
 - a1c78d63ef
 - a73ca336aa
 - 19353017a7
 - b4231844a2
 - 4cb945d780
 - 4ce58df854
 - e2433308c4
 - 013980d839

Both files were contributed while the `COPYING` file for GLib was
LGPL-2.1-or-later, so both were previously implicitly licensed as that.
Let’s make that explicit.

Signed-off-by: Philip Withnall <pwithnall@gnome.org>

Helps: #1415

gio/gdbus-2.0/codegen/.flake8

@@ -1,3 +1,6 @@
+# SPDX-FileCopyrightText: 2022 Emmanuele Bassi
+# SPDX-License-Identifier: LGPL-2.1-or-later
+
 [flake8]
 # We are generating long lines through templates
 max-line-length = 120

gio/gdbus-2.0/codegen/meson.build

@@ -1,3 +1,20 @@
+# Copyright 2016, 2017 Centricular
+#
+# SPDX-License-Identifier: LGPL-2.1-or-later
+#
+# This library 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.
+#
+# This library 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 this library; if not, see <http://www.gnu.org/licenses/>.
+
 gdbus_codegen_files = [
   '__init__.py',
   'codegen.py',