TTY: pty, fix compilation warning

When CONFIG_UNIX98_PTYS is unset, we see this warning in pty:
drivers/tty/pty.c:409:13: warning: ‘pty_unix98_shutdown’ defined but not used

Fix that by moving the function to a section which depends on that
config.

Signed-off-by: Jiri Slaby <[e-mail]>
Reported-by: Toralf Foerster <[e-mail]>
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/tty/pty.c

···
 	return retval;
 }
 
-/* this is called once with whichever end is closed last */
-static void pty_unix98_shutdown(struct tty_struct *tty)
-{
-	devpts_kill_index(tty->driver_data, tty->index);
-}
-
 static void pty_cleanup(struct tty_struct *tty)
 {
 	tty_port_put(tty->port);
···
 
 static void pty_unix98_remove(struct tty_driver *driver, struct tty_struct *tty)
 {
+}
+
+/* this is called once with whichever end is closed last */
+static void pty_unix98_shutdown(struct tty_struct *tty)
+{
+	devpts_kill_index(tty->driver_data, tty->index);
 }
 
 static const struct tty_operations ptm_unix98_ops = {