netfilter: nf_log: add module softdeps

xt_LOG has no direct dependency on the syslog-based logger, it relies
on the nf_log core to probe the requested backend.

Now that all syslog-based loggers reside in the same module, we can
just add a soft dependency on nf_log_syslog and let modprobe take
care of it.

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>