audioconvert: Add missing include for FF_API_AUDIO_CONVERT

Fixes warning in `make checkheaders`.
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/audioconvert.h

@@ -23,6 +23,8 @@
 #ifndef AVCODEC_AUDIOCONVERT_H
 #define AVCODEC_AUDIOCONVERT_H
 
+#include "version.h"
+
 /**
  * @file
  * Audio format conversion routines