netfilter: ip6t_LOG: Fix a typo in a comment

s/_IPT_LOG_H/_IP6T_LOG_H/

While at it add some surrounding space to ease reading.

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>