netfilter: xt_LOG: use CONFIG_IP6_NF_IPTABLES instead of CONFIG_IPV6

This fixes the following linking error:

xt_LOG.c:(.text+0x789b1): undefined reference to `ip6t_ext_hdr'

ifdefs have to use CONFIG_IP6_NF_IPTABLES instead of CONFIG_IPV6.

Acked-by: Randy Dunlap <rdunlap@xenotime.net>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>