audio/audio: Add missing fall through comment

When building with GCC9 using CFLAG -Wimplicit-fallthrough=2 we get:

  audio/audio.c: In function ‘audio_pcm_init_info’:
  audio/audio.c:306:14: error: this statement may fall through [-Werror=implicit-fallthrough=]
    306 |         sign = 1;
        |         ~~~~~^~~
  audio/audio.c:307:5: note: here
    307 |     case AUDIO_FORMAT_U8:
        |     ^~~~
  cc1: all warnings being treated as errors

Similarly to e4634941, add the missing fall through comment to
hint GCC.

Fixes: 2b9cce8c
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Aleksandar Markovic <amarkovic@wavecomp.com>
Reviewed-by: Gerd Hoffmann <kraxel@redhat.com>
Message-Id: <20191218192526.13845-2-philmd@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

audio/audio.c

@@ -304,6 +304,7 @@ void audio_pcm_init_info (struct audio_pcm_info *info, struct audsettings *as)
     switch (as->fmt) {
     case AUDIO_FORMAT_S8:
         sign = 1;
+        /* fall through */
     case AUDIO_FORMAT_U8:
         mul = 1;
         break;