[composer] Fix python bindings for composer attribute table

Author: m-kuhn

python/core/composer/qgscomposerattributetable.sip

@@ -57,7 +57,8 @@ class QgsComposerAttributeTable : QgsComposerTable
 
     //void setSortAttributes( const QList<QPair<int, bool> > att );
     //QList<QPair<int, bool> > sortAttributes() const;
-
+  protected:
+    virtual QMap<int, QString> getHeaderLabels() const;
 
   signals:
     /**This signal is emitted if the maximum number of feature changes (interactively)*/