Fix build against recent sip/PyQt4:

qgsfiledownloader.sip:33:0:
src/gui/qgsfiledownloader.h:94:5: error: overriding non-deleted function 'virtual QgsFileDownloader::~QgsFileDownloader()'

RuntimeError: qgis._core cannot import type 'QList<QVariant>' from PyQt4.QtCore

Author: manisandro

python/core/core.sip

@@ -3,6 +3,7 @@
 
 %Feature QT5_SUPPORT
 
+%Import QtCore/QtCoremod.sip
 %Import QtXml/QtXmlmod.sip
 %Import QtNetwork/QtNetworkmod.sip
 %Import QtSql/QtSqlmod.sip

src/gui/qgsfiledownloader.h

@@ -90,8 +90,10 @@ 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.