Make BindingBase.GetParentDataContext private.

src/Markup/Avalonia.Markup/Data/BindingBase.cs

@@ -234,7 +234,7 @@ namespace Avalonia.Data
             return result;
         }
 
-        protected IObservable<object?> GetParentDataContext(AvaloniaObject target)
+        private IObservable<object?> GetParentDataContext(AvaloniaObject target)
         {
             // The DataContext is based on the visual parent and not the logical parent: this may
             // seem counter intuitive considering the fact that property inheritance works on the logical