TT#83952 Switch Build-Dependency from iptables-dev to libxtables-dev

iptables-dev is only available until Debian/buster and no longer exists in
Debian/bullseye (current testing) nor unstable/sid:

| builddeps:. : Depends: iptables-dev (>= 1.4) but it is not installable

Even in Debian/buster iptables-dev is already a transitional/dummy package.
Support iptables-dev as alternative Build-Dependency, just in case
someone is building the package against a system where libxtables-dev
doesn't exist yet.

Change-Id: I28c4c81ac474c646d80a0146baa2446dde7073c3

debian/control

@@ -8,7 +8,6 @@ Build-Depends:
  debhelper-compat (= 12),
  default-libmysqlclient-dev,
  gperf,
- iptables-dev (>= 1.4),
  libavcodec-dev (>= 6:10),
  libavfilter-dev (>= 6:10),
  libavformat-dev (>= 6:10),
@@ -37,6 +36,7 @@ Build-Depends:
  libswresample-dev (>= 6:10),
  libsystemd-dev,
  libxmlrpc-core-c3-dev (>= 1.16.07),
+ libxtables-dev (>= 1.4) | iptables-dev (>= 1.4),
  markdown,
  zlib1g-dev,