net: add random_port() prototype

The random_port() is meant to be used by other net code, but without a
prototype, we get fun warnings like:
dns.c: In function 'DnsSend':
dns.c:89: warning: implicit declaration of function 'random_port'

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Ben Warren <biggerbadderben@gmail.com>