net/netfilter: fix windows compile error

Resolve compilation errors encountered during compilation in windows
platform.

Signed-off-by: zhanghongyu <zhanghongyu@xiaomi.com>

net/netfilter/ip6t_sockopt.c

@@ -80,6 +80,8 @@ static struct ip6t_table_s g_tables[] =
 {
 #ifdef CONFIG_NET_IPFILTER
   {NULL, ip6t_filter_init, ip6t_filter_apply},
+#else
+  {NULL, NULL, NULL}
 #endif
 };