runtime: Set b:undo_indent where missing (#12944)

Signed-off-by: Christian Brabandt <cb@256bit.org>

runtime/indent/dune.vim

@@ -3,6 +3,7 @@
 " Maintainers:  Markus Mottl         <markus.mottl@gmail.com>
 " URL:          https://github.com/ocaml/vim-ocaml
 " Last Change:  2021 Jan 01
+"               2023 Aug 28 by Vim Project (undo_indent)
 
 if exists("b:did_indent")
  finish
@@ -11,3 +12,5 @@ let b:did_indent = 1
 
 " dune format-dune-file uses 1 space to indent
 setlocal softtabstop=1 shiftwidth=1 expandtab
+
+let b:undo_indent = "setl et< sts< sw<"