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

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

runtime/indent/logtalk.vim

@@ -1,5 +1,6 @@
 "  Maintainer:	Paulo Moura <pmoura@logtalk.org>
 "  Revised on:	2018.08.04
+"		2023 Aug 28 by Vim Project (undo_indent)
 "  Language:	Logtalk
 
 " This Logtalk indent file is a modified version of the Prolog
@@ -16,6 +17,8 @@ setlocal indentexpr=GetLogtalkIndent()
 setlocal indentkeys-=:,0#
 setlocal indentkeys+=0%,-,0;,>,0)
 
+let b:undo_indent = "setlocal indentexpr< indentkeys<"
+
 " Only define the function once.
 if exists("*GetLogtalkIndent")
 	finish