TT#14008 add iptables as explicit runtime dependency

Bullseye doesn't install iptables by default any more, but the included
startup script uses iptables to set up the kernel forwarding by default.
Add an explicit dependency.

closes #1343

Change-Id: I6c222c290e51177f92136f9df59fa769c05ec266
(cherry picked from commit 9376204498)

debian/control

@@ -49,6 +49,7 @@ Recommends:
 Suggests:
  ngcp-system-tools,
 Depends:
+ iptables,
  lsb-base (>= 3.0-6),
  ${misc:Depends},
  ${shlibs:Depends},