runtime: Set b:undo_ftplugin where missing (#12943)

Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/runtime/ftplugin/sexplib.vim b/runtime/ftplugin/sexplib.vim
index 27e1b28..b0767ef 100644
--- a/runtime/ftplugin/sexplib.vim
+++ b/runtime/ftplugin/sexplib.vim
@@ -3,6 +3,7 @@
 " Maintainer:  Markus Mottl        <markus.mottl@gmail.com>
 " URL:         https://github.com/ocaml/vim-ocaml
 " Last Change:
+"              2023 Aug 28 - Added undo_ftplugin (Vim Project)
 "              2017 Apr 12 - First version (MM)
 
 if exists("b:did_ftplugin")
@@ -13,3 +14,5 @@
 " Comment string
 setl commentstring=;\ %s
 setl comments=:;
+
+let b:undo_ftplugin = "setl cms< com<"