iptables: add Suggest: firewalld

Suggest firewalld. Using an higher-level wrapper rather than directly iptables
is in general a good idea I think.

See this debian-devel mailing list thread for context:
 https://lists.debian.org/debian-devel/2019/07/msg00332.html

Signed-off-by: Arturo Borrero Gonzalez <arturo@debian.org>

debian/control

@@ -29,7 +29,7 @@ Depends: libip4tc2 (=${binary:Version}),
          libxtables12 (=${binary:Version}),
          ${misc:Depends},
          ${shlibs:Depends}
-Suggests: kmod
+Suggests: firewalld, kmod
 Breaks: iptables-nftables-compat (<< 1.6.2~)
 Replaces: iptables-nftables-compat (<< 1.6.2~)
 Recommends: nftables