random: Include header where it is used

Unit tests were failing to build because random header was included by
kernel_includes.h. The problem is that rand32.h includes a generated
file that is either not generated or not included when building unit
tests. Also, it is better to limit the scope of this file to where it is
used.

Signed-off-by: Flavio Ceolin <flavio.ceolin@intel.com>