patch 9.0.1829: Vim9 missing access-checks for private vars

Problem:  Vim9 missing access-checks for private vars
Solution: Use the proper check for private/readonly variable.  Access
          level for a member cannot be changed in a class implementing an
          interface.  Update the code indentation

closes: #12978

Signed-off-by: Christian Brabandt <cb@256bit.org>
Co-authored-by: Yegappan Lakshmanan <yegappan@yahoo.com>
Co-authored-by: Ernie Rael <errael@raelity.com>

src/option.c

@@ -2982,7 +2982,8 @@ insecure_flag(int opt_idx, int opt_flags)
 /*
  * Redraw the window title and/or tab page text later.
  */
-void redraw_titles(void)
+    void
+redraw_titles(void)
 {
     need_maketitle = TRUE;
     redraw_tabline = TRUE;