selinux.c: Use same license as upstream file (LGPLv2.1+)

The upstream file is licensed under the LGPLv2.1+ and it uses an SPDX
license identifier rather than an LGPL license notice[1].

And according to the GNU project, the LGPLv2.1+ is compatible with both
the GPLv2 (with the result being GPLv2) and the GPLv3 (with the result
being GPLv3), though the reverse (GPL -> LGPL) does not apply[2] [3].
This means that if we make changes that are only available under the
GPLv2, systemd would be unable to copy them back and release the result
under the LGPLv2.1 without being in violation of the GPLv2.

So replace the GPL license notice with the SPDX license identifier of
the upstream file ("LGPL-2.1-or-later"), to make it easier to share
changes between both projects.

See also the following systemd commits[4] [5] [6] [7]:

* 53e1b68390 ("Add SPDX license identifiers to source files under the
  LGPL", 2017-11-18)
* db9ecf0501 ("license: LGPL-2.1+ -> LGPL-2.1-or-later", 2020-11-09)

[1] https://github.com/systemd/systemd/blob/254d1313ae5a69c08c9b93032aaaf3d6083cfc07/src/shared/selinux-util.c
[2] https://www.gnu.org/licenses/license-list.en.html#LGPLv2.1
[3] https://www.gnu.org/licenses/license-compatibility.html
[4] https://github.com/systemd/systemd/commit/53e1b683907c2f12330f00feb9630150196f064d
[5] https://github.com/systemd/systemd/pull/7386
[6] https://github.com/systemd/systemd/commit/db9ecf050165fd1033c6f81485917e229c4be537
[7] https://github.com/systemd/systemd/pull/17548

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