extensions: libxt_NFLOG: remove extra space when saving targets with prefixes

extensions: libxt_NFLOG: remove extra space when saving targets with prefixes

When printing out NFLOG targets an extra space was inserted between
`--nflog-prefix` and the prefix itself:

  $ sudo /usr/sbin/iptables -A INPUT -j NFLOG --nflog-prefix test
  $ sudo /usr/sbin/iptables-save | grep NFLOG
  -A INPUT -j NFLOG --nflog-prefix  test
                                  ^^
Fixes: 73866357e4a7 ("iptables: do not print trailing whitespaces")
Signed-off-by: Jeremy Sowden <jeremy@azazel.net>
Signed-off-by: Florian Westphal <fw@strlen.de>