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>

include/linux/netfilter/ipset/ip_set.h

@@ -124,8 +124,6 @@ struct ip_set_ext {
 	bool target;
 };
 
-struct ip_set;
-
 #define ext_timeout(e, s)	\
 ((unsigned long *)(((void *)(e)) + (s)->offset[IPSET_EXT_ID_TIMEOUT]))
 #define ext_counter(e, s)	\