batman-adv: Introduce missing headers for genetlink restructure

Fixes: 56989f6d ("genetlink: mark families as __ro_after_init")
Fixes: 2ae0f17d ("genetlink: use idr to track families")
Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>