staging: speakup: fix warning for static declaration

Fix the following sparse warning:
symbol 'spk_serial_out' was not declared. Should it be static?

Signed-off-by: Gustavo A. R. Silva <garsilva@embeddedor.com>
Reviewed-by: Samuel Thibault <samuel.thibault@ens-lyon.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>