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: postfix-policyd-proxy
 Section: unknown
 Priority: optional
 Maintainer: ale <ale@incal.net>
-Build-Depends: debhelper-compat (= 12), dh-golang, golang-any
+Build-Depends: debhelper-compat (= 13), dh-golang, golang-any
 Standards-Version: 4.1.3
 XS-Go-Import-Path: git.autistici.org/ai3/tools/postfix-policyd-proxy