Show a useful dialog when debugger can't find debugging helper.

Reviewed-by: hjk <qtc-committer@nokia.com>

src/plugins/debugger/gdbengine.h

@@ -136,6 +136,7 @@ private:
     //
     // Own stuff
     //
+    void showDebuggingHelperWarning();
     int currentFrame() const;
     QString currentWorkingDirectory() const { return m_pwd; }