extensions: libxt_NFLOG: display nflog-size even if it is zero

extensions: libxt_NFLOG: display nflog-size even if it is zero

The following iptables rules have the different semantics:
  # iptables -A INPUT -j NFLOG
  # iptables -A INPUT -j NFLOG --nflog-size 0

But they are all displayed as "-A INPUT -j NFLOG", so if
the user input the following commands, the original semantics
will be broken.
  # iptables-save | iptables-restore

Signed-off-by: Liping Zhang <liping.zhang@spreadtrum.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>