af_unix: Add a prompt to CONFIG_AF_UNIX_OOB

This makes it possible to disable the MSG_OOB support in .config.

Signed-off-by: Florent Revest <revest@chromium.org>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20241218143334.1507465-1-revest@chromium.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>