batman-adv: Drop unused net_namespace.h include

Since commit 69c7be1b ("rtnetlink: Pack newlink() params into struct"),
.newlink() doesn't use the struct net parameter. A type definition from
net_namespace.h is no longer needed.

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