uapi: install batman_adv.h header

09748a22f4ab ("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>

include/uapi/linux/Kbuild

@@ -64,6 +64,7 @@ header-y += auto_fs.h
 header-y += auxvec.h
 header-y += ax25.h
 header-y += b1lli.h
+header-y += batman_adv.h
 header-y += baycom.h
 header-y += bcm933xx_hcs.h
 header-y += bfs_fs.h