netfilter: nf_conntrack_tuple_common.h: fix #include

To allow usage of enum ip_conntrack_dir in include/net/netns/conntrack.h,
this patch encloses #include <linux/netfilter.h> in a #ifndef __KERNEL__
directive, so that compiler errors caused by unwanted inclusion of
include/linux/netfilter.h are avoided.
In addition, #include <linux/netfilter/nf_conntrack_common.h> line has
been added to resolve correctly CTINFO2DIR macro.

Signed-off-by: Davide Caratti <dcaratti@redhat.com>
Acked-by: Mikko Rapeli <mikko.rapeli@iki.fi>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>