batman-adv: Keep batadv_netlink_notify_* static

The batadv_netlink_notify_*() functions are not used by any other source
file. Just keep them local to netlink.c to get informed by the compiler
when they are not used anymore.

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