patch 8.2.3320: some local functions are not static

Problem:    Some local functions are not static.
Solution:   Add "static".  Move snprintf() related code to strings.c.
            (Yegappan Lakshmanan, closes #8734)

src/spell.c

@@ -1240,7 +1240,7 @@ spell_valid_case(
 /*
  * Return TRUE if spell checking is not enabled.
  */
-    int
+    static int
 no_spell_checking(win_T *wp)
 {
     if (!wp->w_p_spell || *wp->w_s->b_p_spl == NUL