selftests/landlock: Fix build with non-default pthread linking

Old toolchains require explicit -lpthread (e.g. on Debian 11).

Cc: Nathan Chancellor <nathan@kernel.org>
Cc: Tahera Fahimi <fahimitahera@gmail.com>
Fixes: c8994965 ("selftests/landlock: Test signal scoping for threads")
Reviewed-by: Günther Noack <gnoack3000@gmail.com>
Link: https://lore.kernel.org/r/20250115145409.312226-1-mic@digikod.net


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