ffmpeg: do not use SWR_CH_MAX

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

ffmpeg.h

@@ -402,7 +402,8 @@ typedef struct OutputStream {
     double forced_keyframes_expr_const_values[FKF_NB];
 
     /* audio only */
-    int audio_channels_map[SWR_CH_MAX];  /* list of the channels id to pick from the source stream */
+#define FF_CH_MAX 32
+    int audio_channels_map[FF_CH_MAX];   /* list of the channels id to pick from the source stream */
     int audio_channels_mapped;           /* number of channels in audio_channels_map */
 
     char *logfile_prefix;