MAINTAINERS: Mark BATMAN ADVANCED mailing list as moderated

The mailing list of b.a.t.m.a.n@lists.open-mesh.org is moderated for
non-subscribers and non-whitelisted addresses. Such mails will be delayed
but the sender will not be informed about the moderation.

Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Antonio Quartulli <a@unstable.cc>