output: include stdarg.h

output: include stdarg.h

Fixes a build error with alsa-utils when build with a uClibc toolchain:

alsa-utils/host/x86_64-buildroot-linux-uclibc/sysroot/usr/include/alsa/output.h:75:66:
 error: unknown type name ‘va_list’
   75 | int snd_output_vprintf(snd_output_t *output, const char *format, va_list args);
      |                                                                  ^~~~~~~
alsa-utils/host/x86_64-buildroot-linux-uclibc/sysroot/usr/include/alsa/output.h:1:1:
 note: ‘va_list’ is defined in header ‘<stdarg.h>’; did you forget to ‘#include <stdarg.h>’?

Fixes: https://github.com/alsa-project/alsa-lib/pull/237
Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Jaroslav Kysela <perex@perex.cz>