chore(sterni/mblog): relicense to GPL-3.0-only

This is possible since all the commits have been made by me. The code
taken from  SCLF (which is licensed LGPL-2.1-or-later) can also be
included since the LGPL 2.1 is [compatible] with the GPL 3.0.

compatible: https://www.gnu.org/licenses/license-list.en.html#LGPLv2.1
Change-Id: I2d274c29378679c489dc667a53b234642c3da817
Reviewed-on: https://cl.tvl.fyi/c/depot/+/5928
Tested-by: BuildkiteCI
Reviewed-by: sterni <sternenseemann@systemli.org>

users/sterni/mblog/transformer.lisp

@@ -1,3 +1,6 @@
+;; SPDX-License-Identifier: GPL-3.0-only
+;; SPDX-FileCopyrightText: Copyright (C) 2022 by sterni
+
 (in-package :note)
 (declaim (optimize (safety 3)))