batman-adv: Let packet.h include its headers directly

The headers used by packet.h should also be included by it directly. main.h
is currently dealing with it in batman-adv, but this will no longer work
when this header is moved to include/uapi/linux/.

Signed-off-by: Sven Eckelmann <sven.eckelmann@openmesh.com>
Signed-off-by: David S. Miller <davem@davemloft.net>