avdevice/alsa: add av_warn_unused_result

This does not trigger any warnings, but adds robustness.
Reviewed-by: Nicolas George <george@nsup.org>
Signed-off-by: Ganesh Ajjanagadde <gajjanagadde@gmail.com>

libavdevice/alsa.h

@@ -74,6 +74,7 @@ typedef struct AlsaData {
  *
  * @return 0 if OK, AVERROR_xxx on error
  */
+av_warn_unused_result
 int ff_alsa_open(AVFormatContext *s, snd_pcm_stream_t mode,
                  unsigned int *sample_rate,
                  int channels, enum AVCodecID *codec_id);
@@ -95,10 +96,13 @@ int ff_alsa_close(AVFormatContext *s1);
  *
  * @return 0 if OK, AVERROR_xxx on error
  */
+av_warn_unused_result
 int ff_alsa_xrun_recover(AVFormatContext *s1, int err);
 
+av_warn_unused_result
 int ff_alsa_extend_reorder_buf(AlsaData *s, int size);
 
+av_warn_unused_result
 int ff_alsa_get_device_list(AVDeviceInfoList *device_list, snd_pcm_stream_t stream_type);
 
 #endif /* AVDEVICE_ALSA_H */