batman: fix duplicate #include of multicast.h

The header multicast.h was included twice, so delete one of them.

Signed-off-by: Jean Sacren <sakiwit@gmail.com>
Cc: Marek Lindner <mareklindner@neomailbox.ch>
Cc: Simon Wunderlich <sw@simonwunderlich.de>
Cc: Antonio Quartulli <antonio@meshcoding.com>
Cc: b.a.t.m.a.n@lists.open-mesh.org
Signed-off-by: David S. Miller <davem@davemloft.net>