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>

src/window.c

@@ -97,14 +97,14 @@ static int close_disallowed = 0;
  * make sure the previously selected window is still there.
  * Must be matched with exactly one call to window_layout_unlock()!
  */
-    static void
+    void
 window_layout_lock(void)
 {
     ++split_disallowed;
     ++close_disallowed;
 }
 
-    static void
+    void
 window_layout_unlock(void)
 {
     --split_disallowed;