wxOSX compilation fix after wxDataViewRendererBase refactoring

Restore the variable declaration accidentally removed in the changes of
235e8ebd1a

src/osx/cocoa/dataview.mm

@@ -1886,6 +1886,9 @@ outlineView:(NSOutlineView*)outlineView
     wxDataViewColumn* const
         col([static_cast<wxDVCNSTableColumn*>(tableColumn) getColumnPointer]);
 
+    wxDataViewCtrl* const dvc = implementation->GetDataViewCtrl();
+
+
     // stop editing of a custom item first (if necessary)
     dvc->FinishCustomItemEditing();