batman-adv: Switch to linux/sprintf.h

The commit 39ced19b ("lib/vsprintf: split out sprintf() and friends")
introduced a new header for the sprintf related functions which were
previously exposed via linux/kernel.h.

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