Debugger: Re-enable thread combo box for core files

This fixes a regression introduced after 2.5

Task-number: QTCREATORBUG-9110
Change-Id: I72450d7c6797910e789f78c7478fc7dd3d6f6228
Reviewed-by: Eike Ziller <eike.ziller@digia.com>

src/plugins/debugger/debuggerplugin.cpp

@@ -2311,7 +2311,7 @@ void DebuggerPluginPrivate::updateState(DebuggerEngine *engine)
     m_attachToRemoteServerAction->setEnabled(true);
     m_attachToRunningApplication->setEnabled(true);
 
-    m_threadBox->setEnabled(state == InferiorStopOk);
+    m_threadBox->setEnabled(state == InferiorStopOk || state == InferiorUnrunnable);
 
     const bool isCore = engine->startParameters().startMode == AttachCore;
     const bool stopped = state == InferiorStopOk;