Set debhelper-compat version in Build-Depends.

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

debian/compat

-12

debian/control

@@ -2,7 +2,8 @@ Source: rsyslog-exporter
 Section: admin
 Priority: optional
 Maintainer: Autistici/Inventati <debian@autistici.org>
-Build-Depends: debhelper (>= 12~), golang-any, dh-golang,
+Build-Depends: debhelper-compat (= 12),
+ golang-any, dh-golang,
  golang-github-prometheus-client-golang-dev
 Standards-Version: 3.9.6