dec_audio: add missing include

Was masked by FFmpeg's terrible headers, but failed with Libav.

audio/decode/dec_audio.c

@@ -18,6 +18,7 @@
 #include <stdio.h>
 #include <stdlib.h>
 #include <unistd.h>
+#include <math.h>
 #include <assert.h>
 
 #include <libavutil/mem.h>