batman-adv: Drop unused header preempt.h

The commit b1de0f01 ("batman-adv: Use netif_rx_any_context().") removed
the last user for a function declaration from linux/preempt.h. The include
should therefore be cleaned up.

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