Bump debhelper from old 12 to 13.

Changes-By: lintian-brush
Fixes: lintian: package-uses-old-debhelper-compat-version
See-also: https://lintian.debian.org/tags/package-uses-old-debhelper-compat-version.html

debian/control

@@ -2,7 +2,7 @@ Source: firewall
 Section: net
 Priority: optional
 Maintainer: Autistici/Inventati <debian@autistici.org>
-Build-Depends: debhelper-compat (= 12)
+Build-Depends: debhelper-compat (= 13)
 Standards-Version: 3.8.0.1
 
 Package: firewall