d/changelog: upload is for experimental rather than unstable

This upload is for Debian experimental rather than unstable.
Suggested-by: Ana Guerrero <ana@debian.org>
Signed-off-by: Arturo Borrero Gonzalez <arturo.borrero.glez@gmail.com>

debian/changelog

-iptables (1.6.0-1) unstable; urgency=medium
+iptables (1.6.0-1) experimental; urgency=medium
 
   * [615f9fc] Imported Upstream version 1.6.0
   * [7caa8f4] d/patches/0104-lintian_hyphens.patch: refresh patch