selftests/landlock: Locally define __maybe_unused

The checkpatch tool started to flag __attribute__(__unused__), which
we previously used. The header where this is normally defined is not
currently compatible with selftests.

This is the same approach as used in selftests/net/psock_lib.h.

Signed-off-by: Günther Noack <gnoack3000@gmail.com>
Link: https://lore.kernel.org/r/20221018182216.301684-8-gnoack3000@gmail.com


Signed-off-by: Mickaël Salaün <mic@digikod.net>