batman-adv: Remove unused sched.h include

The linux/wait.h include was removed with commit 421d988b2c08 ("batman-adv:
Consolidate logging related functions"). The previously required (but not
unused) linux/sched.h include can also be dropped now.

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