netfilter: fix include guards.

nf_conntrack_labels.h has no include guard.  Add it.

The comment following the #endif in the nf_flow_table.h include guard
referred to the wrong macro.  Fix it.

Signed-off-by: Jeremy Sowden <jeremy@azazel.net>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>