package/alsa-lib: fix musl-related openal build error

openal will fail to build without this patch when using pcm.h

In file included from /home/bernd/buildroot/br3/output/host/usr/x86_64-buildroot-linux-musl/sysroot/usr/include/alsa/asoundlib.h:54:0,
                 from /home/bernd/buildroot/br3/output/build/openal-1.17.2/Alc/backends/alsa.c:34:
/home/bernd/buildroot/br3/output/host/usr/x86_64-buildroot-linux-musl/sysroot/usr/include/alsa/pcm.h:1111:1: error: unknown type name ‘u_int8_t’
 u_int8_t snd_pcm_format_silence(snd_pcm_format_t format);
 ^

It seems that this problem was not yet caught by the autobuilders.

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>