Promote iptables from Recommend to Depends

Otherwise tools like grml-router don't work OOTB:

| grml@grml ~ % sudo grml-router
| iptables: binary not found

Given that iptables and its dependencies require less than 3MB of disk
space on e.g. grml64-small, let's add it as dependency.

Closes: https://github.com/grml/grml/issues/196