movenc: add m4b to ipod format extensions

m4b is commonly used as extension for m4a audiobook files.
The format is exactly the same. The only thing that differs
is the extension.
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/movenc.c

@@ -6664,7 +6664,7 @@ AVOutputFormat ff_ipod_muxer = {
     .name              = "ipod",
     .long_name         = NULL_IF_CONFIG_SMALL("iPod H.264 MP4 (MPEG-4 Part 14)"),
     .mime_type         = "video/mp4",
-    .extensions        = "m4v,m4a",
+    .extensions        = "m4v,m4a,m4b",
     .priv_data_size    = sizeof(MOVMuxContext),
     .audio_codec       = AV_CODEC_ID_AAC,
     .video_codec       = AV_CODEC_ID_H264,