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>