ALSA:: multisound.sh: fix script to make it build with modern tools

The script is old and produce some warnings and errors, because
it lacks including stdlib.h and io.h is at sys/io.h.

Fix it to run with the tools found on modern Linux distros.

Tested building it on Fedora 28.

Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>