netfilter: ipset: avoid use of kernel-only types

When using the xt_set.h header in userspace, one will get these gcc
reports:

ipset/ip_set.h:184:1: error: unknown type name "u16"
In file included from libxt_SET.c:21:0:
netfilter/xt_set.h:61:2: error: unknown type name "u32"
netfilter/xt_set.h:62:2: error: unknown type name "u32"

Signed-off-by: Jan Engelhardt <[e-mail]>
Signed-off-by: Jozsef Kadlecsik <[e-mail]>
Signed-off-by: Pablo Neira Ayuso <[e-mail]>

include/linux/netfilter/xt_set.h

···
 struct xt_set_info_target_v2 {
 	struct xt_set_info add_set;
 	struct xt_set_info del_set;
-	u32 flags;
-	u32 timeout;
+	__u32 flags;
+	__u32 timeout;
 };
 
 #endif /*_XT_SET_H*/