V4L/DVB (4611): Add module-init-tools version comment.

I've just determined that module-init-tools >=3.2 is needed for dvb_attach
to work. This adds a comment to Kconfig about it.

Signed-off-by: Andrew de Quincey <adq_dvb@lidskialf.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>