selftests/landlock: Enable the new CONFIG_AF_UNIX_OOB

Since commit 5155cbcd ("af_unix: Add a prompt to
CONFIG_AF_UNIX_OOB"), the Landlock selftests's configuration is not
enough to build a minimal kernel.  Because scoped_signal_test checks
with the MSG_OOB flag, we need to enable CONFIG_AF_UNIX_OOB for tests:

 #  RUN           fown.no_sandbox.sigurg_socket ...
 # scoped_signal_test.c:420:sigurg_socket:Expected 1 (1) == send(client_socket, ".", 1, MSG_OOB) (-1)
 # sigurg_socket: Test terminated by assertion
 #          FAIL  fown.no_sandbox.sigurg_socket
 ...

Cc: Günther Noack <gnoack@google.com>
Acked-by: Florent Revest <revest@chromium.org>
Link: https://lore.kernel.org/r/20250211132531.1625566-1-mic@digikod.net


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