mythfilldatabase: print reason for foo message

Trivial change, but the issue comes up on the mailing list and forum
enough that explaining "No channels are configured" is the result of
not having any XMLTVIDs.

mythtv/programs/mythfilldatabase/filldata.cpp

@@ -346,7 +346,7 @@ bool FillData::Run(DataSourceList &sourcelist)
             else
             {
                 LOG(VB_GENERAL, LOG_INFO,
-                    QString("No channels are configured to use grabber."));
+                    QString("No channels are configured to use grabber (none have XMLTVIDs)."));
             }
         }
         else