Set debhelper-compat version in Build-Depends.

Fixes lintian: uses-debhelper-compat-file
See https://lintian.debian.org/tags/uses-debhelper-compat-file.html for more details.

debian/compat

@@ -1 +0,0 @@
-12

debian/control

@@ -7,7 +7,7 @@ Homepage: https://github.com/mika/jenkins-debian-glue
 Vcs-Git: https://github.com/mika/jenkins-debian-glue.git
 Vcs-Browser: https://github.com/mika/jenkins-debian-glue
 Build-Depends:
- debhelper (>= 12~),
+ debhelper-compat (= 12),
  shunit2,
 
 Package: jenkins-debian-glue