patch 9.1.1323: b:undo_ftplugin not executed when re-using buffer

Problem:  b:undo_ftplugin not executed when re-using buffer
          (archy3)
Solution: explicitly execute b:undo_ftplugin in buflist_new() when
          re-using the current buffer

fixes: #17113
closes: #17133

Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/proto/window.pro b/src/proto/window.pro
index ccb69ef..d92a5af 100644
--- a/src/proto/window.pro
+++ b/src/proto/window.pro
@@ -1,4 +1,6 @@
 /* window.c */
+void window_layout_lock(void);
+void window_layout_unlock(void);
 int window_layout_locked(enum CMD_index cmd);
 int check_can_set_curbuf_disabled(void);
 int check_can_set_curbuf_forceit(int forceit);