staging: speakup: serialio: fix warning linux/serial.h is included more than once

fix below warning reported by  includecheck

./drivers/staging/speakup/serialio.h: linux/serial.h is included more
than once.

Signed-off-by: Hariprasad Kelam <hariprasad.kelam@gmail.com>
Reviewed-by: Samuel Thibault <samuel.thibault@ens-lyon.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>