random32: assign to network folks in MAINTAINERS

lib/random32.c was split out of the network code and is de-facto
still maintained by the almighty net/ gods.

Make it a bit more official so that people who aren't aware of
that know where to send their patches.

Signed-off-by: Sasha Levin <sasha.levin@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>