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

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

runtime/indent/raku.vim

@@ -4,6 +4,7 @@
 " Homepage:      https://github.com/vim-perl/vim-perl
 " Bugs/requests: https://github.com/vim-perl/vim-perl/issues
 " Last Change:   2020 Apr 15
+"                2023 Aug 28 by Vim Project (undo_indent)
 " Contributors:  Andy Lester <andy@petdance.com>
 "                Hinrik Örn Sigurðsson <hinrik.sig@gmail.com>
 "
@@ -47,6 +48,8 @@ if !b:indent_use_syntax
     setlocal indentkeys+=0=EO
 endif
 
+let b:undo_indent = "setlocal indentexpr< indentkeys<"
+
 let s:cpo_save = &cpo
 set cpo-=C