iptables: fix ip6tables-apply manpage symlink

When building the package:

[...]
   dh_install
   dh_installdocs
   dh_installchangelogs
   dh_installman
   dh_perl
   dh_link
   dh_strip_nondeterminism
   dh_compress
dh_compress: symlink(iptables-apply.8.gz, debian/iptables/usr/share/man/man8/ip6tables-apply.8.gz) failed: File exists
dh_compress: Aborting due to earlier error
debian/rules:10: recipe for target 'binary' failed
make: *** [binary] Error 255
dpkg-buildpackage: error: fakeroot debian/rules binary gave error exit status 2
[...]
Signed-off-by: Arturo Borrero Gonzalez <arturo@debian.org>

debian/iptables.links

 /sbin/xtables-multi /usr/bin/iptables-xml
 /usr/sbin/iptables-apply /usr/sbin/ip6tables-apply
-/usr/share/man/man8/iptables-apply.8.gz /usr/share/man/man8/ip6tables-apply.8.gz