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/job.c

@@ -1582,7 +1582,7 @@ invoke_prompt_interrupt(void)
 /*
  * Return the effective prompt for the specified buffer.
  */
-    char_u *
+    static char_u *
 buf_prompt_text(buf_T* buf)
 {
     if (buf->b_prompt_text == NULL)