dbus: Also adjust template.

This is a follow up commit to
d5021c78.

* bin/dbus/cx.ring.Ring.service.in (Exec): Replace @libexec@ by
@libexecdir@.

Change-Id: Id72503d45f24bf1ff4341d025b9c7af51cb52479

bin/dbus/cx.ring.Ring.service.in

 [D-BUS Service]
 Name=cx.ring.Ring
-Exec=@libexec@/jamid
+Exec=@libexecdir@/jamid