QCalendar: actually remember if the registry has been populated

The "populated" variable is otherwise never written into.

Change-Id: I979411a19927dc4e7e09c6c36edfb2308f519596
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/time/qcalendar.cpp

@@ -123,6 +123,8 @@ struct Registry {
 
         for (int i = 0; i <= int(QCalendar::System::Last); ++i)
             (void)QCalendar(QCalendar::System(i));
+
+        populated = true;
     }
 };