MT#60916 lintian: ignore service-file-is-not-a-file for mediator.service

We ship /lib/systemd/system/mediator.service as symlink pointing
to ngcp-mediator.service, ignore in lintian as reported by
lintian v2.118.1 as present in current Debian/testing AKA trixie.

Fixes:

| E: ngcp-mediator: service-file-is-not-a-file [lib/systemd/system/mediator.service]

Change-Id: Ib27cfddefaeb95f52703f695e938736e91fc9939

debian/ngcp-mediator.lintian-overrides

@@ -1,2 +1,3 @@
 ngcp-mediator: binary-without-manpage usr/bin/ngcp-mediator
 ngcp-mediator: init.d-script-not-included-in-package etc/init.d/ngcp-mediator
+ngcp-mediator: service-file-is-not-a-file [lib/systemd/system/mediator.service]