batman-adv: Add module alias for batadv netlink family

The batman-adv module has to be loaded to fulfill genl request by the
userspace. When it is not loaded then requests will fail. It is therefore
useful to get the module automatically loaded when such a request is made.

Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>