Changes

debian/postinst: don't change /etc/osmocom

osmo_dia2gsup doesn't have a config file in /etc/osmocom. The path I put
there didn't make sense.

Fix for:
  chown: cannot access '/etc/osmocom/osmo_dia2gsup.service': No such file or directory

Fixes: 240b43f3 ("contrib/systemd: run as osmocom user")
Change-Id: I32a06db20c53fdc136989f41805c78e5d9420b9b