iptables: bump debhelper-compat to 13

This is the recommended mode of operation.
Signed-off-by: Arturo Borrero Gonzalez <arturo@debian.org>

debian/control

@@ -8,7 +8,7 @@ Uploaders: Arturo Borrero Gonzalez <arturo@debian.org>,
 Build-Depends: autoconf,
                automake,
                bison,
-               debhelper-compat (= 12),
+               debhelper-compat (= 13),
                flex,
                libmnl-dev,
                libnetfilter-conntrack-dev,