netfilter: xt_NFLOG is dependant of nfnetlink_log

The patch "don't call nf_log_packet in NFLOG module" make xt_NFLOG
dependant of nfnetlink_log. This patch forces the dependencies to fix
compilation in case only xt_NFLOG compilation was asked and modifies the
help message accordingly to the change.

Signed-off-by: Eric Leblond <eric@inl.fr>
Signed-off-by: David S. Miller <davem@davemloft.net>