Add wxDataViewCtrl::SetHeaderAttr() too

This is currently only implemented in the generic version but could be
implemented at least for GTK+ native one as well in the future.

include/wx/generic/dataview.h

@@ -256,6 +256,8 @@ public:
 
     virtual void EditItem(const wxDataViewItem& item, const wxDataViewColumn *column) wxOVERRIDE;
 
+    virtual bool SetHeaderAttr(const wxItemAttr& attr) wxOVERRIDE;
+
     // These methods are specific to generic wxDataViewCtrl implementation and
     // should not be used in portable code.
     wxColour GetAlternateRowColour() const { return m_alternateRowColour; }