Fix build against recent sip:

manisandro · manisandro · commit fcf718ff3c99 · 2017-03-01T23:43:09.000+01:00
qgsfiledownloader.sip:33:0:
src/gui/qgsfiledownloader.h:94:5: error: overriding non-deleted function 'virtual QgsFileDownloader::~QgsFileDownloader()'
diff --git a/src/gui/qgsfiledownloader.h b/src/gui/qgsfiledownloader.h
@@ -95,9 +95,12 @@ class GUI_EXPORT QgsFileDownloader : public QObject
      */
     void onSslErrors( QNetworkReply *reply, const QList<QSslError> &errors );
 #endif
-  private:
+
+  protected:
     ~QgsFileDownloader();
 
+  private:
+
     /**
      * Abort current request and show an error if the instance has GUI
      * notifications enabled.
