Fix build (again...)

Author: Gbury

middle_end/flambda/types/structures/row_like.rec.ml

@@ -630,7 +630,7 @@ struct
 
     let _get_variant_field t variant_tag field_index : _ Or_unknown_or_bottom.t =
       let index = Target_imm.to_targetint field_index in
-      let aux { index = size; maps_to; } : _ Or_unknown_or_bottom.t =
+      let aux { index = size; maps_to; env_extension = _; } : _ Or_unknown_or_bottom.t =
         match size with
         | Known i when i <= index -> Bottom
         | _ ->