ocamlPackages.dum: fix build with OCaml ≥ 5.0

pkgs/development/ocaml-modules/dum/default.nix

···
     sha256 = "0yrxl97szjc0s2ghngs346x3y0xszx2chidgzxk93frjjpsr1mlr";
   };
 
+  postPatch = ''
+      substituteInPlace "dum.ml" \
+      --replace "Lazy.lazy_is_val" "Lazy.is_val" \
+      --replace "Obj.final_tag" "Obj.custom_tag"
+  '';
+
   nativeBuildInputs = [ ocaml findlib ];
   propagatedBuildInputs = [ easy-format ];