netfilter: ipset: move ip_set_get_ip_port() to ip_set_bitmap_port.c.

ip_set_get_ip_port() is only used in ip_set_bitmap_port.c.  Move it
there and make it static.

Signed-off-by: Jeremy Sowden <jeremy@azazel.net>
Acked-by: Jozsef Kadlecsik <kadlec@netfilter.org>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>