netfilter: remove stale declaration for ip6_masked_addrcmp()

Commit f2ffd9ee... ("[NETFILTER]: Move ip6_masked_addrcmp to
include/net/ipv6.h") replaced ip6_masked_addrcmp() with
ipv6_masked_addr_cmp().  Function definition went away.

Let's remove its declaration as well in header file.
Signed-off-by: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>
Signed-off-by: Patrick McHardy <kaber@trash.net>

include/linux/netfilter_ipv6/ip6_tables.h

@@ -316,10 +316,6 @@ extern int ip6t_ext_hdr(u8 nexthdr);
 extern int ipv6_find_hdr(const struct sk_buff *skb, unsigned int *offset,
 			 int target, unsigned short *fragoff);
 
-extern int ip6_masked_addrcmp(const struct in6_addr *addr1,
-			      const struct in6_addr *mask,
-			      const struct in6_addr *addr2);
-
 #define IP6T_ALIGN(s) XT_ALIGN(s)
 
 #ifdef CONFIG_COMPAT