net.reactivated.Fprint: Only allow clients to send messages with fprintd iface to us
In the way the rule is currently set it would allow clients to send messages with the fprintd interface to any other service, while we only allow them to be redirected to fprintd itself.
This was causing a debian linter failure [1].
[1] https://lintian.debian.org/tags/dbus-policy-without-send-destination.html