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 <jslaby@suse.cz>
Reported-by: Toralf Foerster <toralf.foerster@gmx.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>