avcodec: fix avutil version check

ffmpeg/libav doc/APIchanges should not be relied upon
(cherry picked from commit 7f510436)
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>

modules/codec/avcodec/audio.c

@@ -41,7 +41,7 @@
 #   include <avcodec.h>
 #endif
 
-#if LIBAVUTIL_VERSION_INT >= ((52<<16)+(38<<8)+0)
+#if LIBAVUTIL_VERSION_INT >= ((50<<16)+(38<<8)+0)
 # include "libavutil/audioconvert.h"
 #endif