bridge: fix IPV6=n build

Fix bridge netfilter code so that it uses CONFIG_IPV6 as needed:

net/built-in.o: In function `ebt_filter_ip6':
ebt_ip6.c:(.text+0x87c37): undefined reference to `ipv6_skip_exthdr'
net/built-in.o: In function `ebt_log_packet':
ebt_log.c:(.text+0x88dee): undefined reference to `ipv6_skip_exthdr'
make[1]: *** [.tmp_vmlinux1] Error 1

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>