[PATCH] On-Demand relay mount doesn't has on_demand set on startup

A <relay> that has the on-demand option set is missing this information on icecast's first startup, when looking at the stats xml

<source mount="/diff.mp3">
    <listener_peak>0</listener_peak>
    <listeners>0</listeners>
    <listenurl>http://localhost:8000/diff.mp3</listenurl>
    <max_listeners>unlimited</max_listeners>
    <public>0</public>
</source>

missing the <on_demand>1</on_demand> on first startup. When a listener connects and it therefore connects to the relayed stream, it will have this set. The patch makes sure that this is set initially too.