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

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

runtime/indent/rst.vim

@@ -4,6 +4,7 @@
 " Maintainer: Marshall Ward <marshall.ward@gmail.com>
 " Previous Maintainer: Nikolai Weibull <now@bitwi.se>
 " Latest Revision: 2020-03-31
+"                  2023 Aug 28 by Vim Project (undo_indent)
 
 if exists("b:did_indent")
   finish
@@ -14,6 +15,8 @@ setlocal indentexpr=GetRSTIndent()
 setlocal indentkeys=!^F,o,O
 setlocal nosmartindent
 
+let b:undo_indent = "setlocal indentexpr< indentkeys< smartindent<"
+
 if exists("*GetRSTIndent")
   finish
 endif