MAINTAINERS: ivtv-devel is moderated

Mark ivtv-devel@ivtvdriver.org as 'moderated for non-subscribers'.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Acked-by: Andy Walls <awalls@radix.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>