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: NJiri Slaby <jslaby@suse.cz>
Reported-by: NToralf Foerster <toralf.foerster@gmx.de>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/pty.c

@@ -405,12 +405,6 @@ static int pty_common_install(struct tty_driver *driver, struct tty_struct *tty,
 	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);
@@ -626,6 +620,12 @@ 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 = {
 	.lookup = ptm_unix98_lookup,
 	.install = pty_unix98_install,