batman-adv: Drop prandom.h includes

The commit 8032bf12 ("treewide: use get_random_u32_below() instead of
deprecated function") replaced the prandom.h function prandom_u32_max with
the random.h function get_random_u32_below. There is no need to still
include prandom.h.

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