Add missing binding

Author: nyalldawson

python/gui/editorwidgets/qgsrelationreferencewidget.sip

@@ -71,6 +71,11 @@ class QgsRelationReferenceWidget : QWidget
     //! if no feature is related, it returns an invalid feature
     QgsFeature referencedFeature();
 
+    /** Sets the widget to display in an indeterminate "mixed value" state.
+     * @note added in QGIS 2.16
+     */
+    void showIndeterminateState();
+
   public slots:
     //! open the form of the related feature in a new dialog
     void openForm();