uapi: install batman_adv.h header

09748a22 ("batman-adv: add generic netlink family for batman-adv")
introduced the new batman_adv.h which describes the netlink attributes and
commands of batman-adv. But the Kbuild entry to install the header was not
added.

All currently known tools ship their own copy of batman_adv.h but it should
be installed anyway to later be able to migrate to the system batman_adv.h.

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