avutil/audio_fifo: split into a separate doxy module

Signed-off-by: Timothy Gu <timothygu99@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavutil/audio_fifo.h

@@ -34,6 +34,9 @@
 /**
  * @addtogroup lavu_audio
  * @{
+ *
+ * @defgroup lavu_audiofifo Audio First-In-First-Out Buffer
+ * @{
  */
 
 /**
@@ -143,6 +146,7 @@ int av_audio_fifo_size(AVAudioFifo *af);
 int av_audio_fifo_space(AVAudioFifo *af);
 
 /**
+ * @}
  * @}
  */