netfilter: ipset: Remove duplicate declaration

struct ip_set is declared twice. One is declared at 79th line,
so remove the duplicate.

Signed-off-by: Wan Jiabing <wanjiabing@vivo.com>
Acked-by: Jozsef Kadlecsik <kadlec@netfilter.org>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>