Handle wxDF_PNG in wxDataViewCtrlBase::CreateDataObject() too

If nothing else, this avoids warnings about missing enum element in the
switch.

src/common/datavcmn.cpp

@@ -1709,6 +1709,7 @@ wxDataObject* wxDataViewCtrlBase::CreateDataObject(const wxDataFormatArray& form
                 break;
 
             case wxDF_BITMAP:
+            case wxDF_PNG:
                 dataObject->Add(new wxBitmapDataObject);
                 break;