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

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>