mcheck: Fix SPDX License Identifier to LGPL-2.1-or-later

The GPL-2.1+ SPDX License Identifier doesn't exist, but luckily the full
license text was available which shows that GPL-2.1+ was a typo and it
should have been LGPL-2.1-or-later.
As the '+' in LGPL-2.1+ is deprecated in SPDX 3.0, use the preferred
identifier, which is '-or-later'.

Normally the full license header is removed when switching to SPDX
License Identifiers, so do that now.

Signed-off-by: Diederik de Haas <didi.debian@cknow.org>

include/mcheck.h

@@ -1,18 +1,7 @@
-/* SPDX-License-Identifier: GPL-2.1+ */
+/* SPDX-License-Identifier: LGPL-2.1-or-later */
 /*
  * Copyright (C) 1996-2024 Free Software Foundation, Inc.
  * This file is part of the GNU C Library.
- *
- * 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.
- * <https://www.gnu.org/licenses/>.
  */
 #ifndef _MCHECK_H
 #define _MCHECK_H       1