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

drivers/staging/speakup/serialio.c

@@ -243,7 +243,7 @@ unsigned char spk_serial_in_nowait(void)
 }
 EXPORT_SYMBOL_GPL(spk_serial_in_nowait);
 
-int spk_serial_out(struct spk_synth *in_synth, const char ch)
+static int spk_serial_out(struct spk_synth *in_synth, const char ch)
 {
 	if (in_synth->alive && spk_wait_for_xmitr(in_synth)) {
 		outb_p(ch, speakup_info.port_tts);