batadv_gw_modes and BATADV_ATTR_GW_MODE were missing from the local
version, so update it and the corresponding policy table from batctl.

Interacting with batman-adv's genl interface requires some code and
definitions which could be shared between different packages. libbatadv is
trying to do this without providing any guarantee for ABI or API stability.
It is only useful in very controlled environments like gluon.

Signed-off-by: Sven Eckelmann <sven@narfation.org>