netfilter: ipset: make ip_set_put_flags extern.

ip_set_put_flags is rather large for a static inline function in a
header-file.  Move it to ip_set_core.c and export it.

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