batman-adv: Migrate to linux/prandom.h

The commit c0842fbc ("random32: move the pseudo-random 32-bit
definitions to prandom.h") introduced a new header for the pseudo random
functions from (previously) linux/random.h. One future goal of the
prandom.h change is to make code to switch just the new header file and to
avoid the implicit include. This would allow the removal of the implicit
include from random.h

Signed-off-by: Sven Eckelmann <sven@narfation.org>