Update pkgr.yml for consensisty with Ubuntu 18.04

This patch updates pkgr.yml as it seems to be inconsistent.
Ubuntu 18.04 is currently the only system missing "- libimlib2-dev" for installation dependency and "- libimlib2" for build dependency.
This looks wrong.

If you don't install libimlib2-dev manually, zammad will not start cleanly.

.pkgr.yml

@@ -56,7 +56,9 @@ targets:
       - nginx|apache2
       - postgresql|mysql-server|mariadb-server|sqlite
       - libimlib2
+      - libimlib2-dev
     build_dependencies:
+      - libimlib2
       - libimlib2-dev
   sles-12:
     dependencies: