Fix: USD: Don't attempt to export the special custom_normal attribute

Suppress exporting this attribute as there's no conversion to USD that
supports its data type (2d, 16-bit int) and because mesh normals (from
the `corner_normals` API) are already exported. This causes a rather
annoying UI Warning notification that the attribute isn't convertible
during export.

Pull Request: https://projects.blender.org/blender/blender/pulls/135271

source/blender/io/usd/intern/usd_writer_mesh.cc

@@ -157,7 +157,7 @@ void USDGenericMeshWriter::write_custom_data(const Object *obj,
      * Skip edge domain because USD doesn't have a good conversion for them. */
     if (iter.name[0] == '.' || bke::attribute_name_is_anonymous(iter.name) ||
         iter.domain == bke::AttrDomain::Edge ||
-        ELEM(iter.name, "position", "material_index", "velocity", "crease_vert"))
+        ELEM(iter.name, "position", "material_index", "velocity", "crease_vert", "custom_normal"))
     {
       return;
     }