Make the free space updates start in PBH

Make it so as soon as the playbackboxhelper thread starts, it forces an update
of the free space so the bar can start updating.  Once it has been started,
the free space will update on a timer basis (looks like every 15s).

Fixes #9771
(cherry picked from commit 62d7084)

mythtv/programs/mythfrontend/playbackboxhelper.cpp

@@ -520,6 +520,8 @@ void PlaybackBoxHelper::UndeleteRecording(
 void PlaybackBoxHelper::run(void)
 {
     m_eventHandler = new PBHEventHandler(*this);
+    // Prime the pump so the disk free display starts updating
+    ForceFreeSpaceUpdate();
     exec();
 }