debian: fix postinst script not executing systemd actions

Change-Id: Ia336cb10a2617312c95e5e3d7fbcd38674fa4519
Reviewed-on: https://forge.frm2.tum.de/review/c/frm2/tango/entangle/+/26341

Tested-by: Georg Brandl <g.brandl@fz-juelich.de>
Reviewed-by: Georg Brandl <g.brandl@fz-juelich.de>

debian/postinst

 #!/bin/sh
 set -e
 journalctl --update-catalog
+
+#DEBHELPER#
+
+exit 0