fix build error on precise

Author: jef-n

src/gui/qgisgui.h

@@ -83,7 +83,7 @@ namespace QgisGui
    * the file type
    * @note added in 1.6
    */
-  QPair<QString, QString> GUI_EXPORT getSaveAsImageName( QWidget * theParent, QString theMessage, QString defaultFilename = NULL );
+  QPair<QString, QString> GUI_EXPORT getSaveAsImageName( QWidget * theParent, QString theMessage, QString defaultFilename = QString::null );
 
   /**