lavf/dashdec: Add webm to the list of allowed extensions.

This is in coherence with dashenc, which can now generate segments with
webm file name extension by default. Dashdec should be able to handle
such streams by default as well.

libavformat/dashdec.c

@@ -2242,7 +2242,7 @@ static int dash_probe(AVProbeData *p)
 static const AVOption dash_options[] = {
     {"allowed_extensions", "List of file extensions that dash is allowed to access",
         OFFSET(allowed_extensions), AV_OPT_TYPE_STRING,
-        {.str = "aac,m4a,m4s,m4v,mov,mp4"},
+        {.str = "aac,m4a,m4s,m4v,mov,mp4,webm"},
         INT_MIN, INT_MAX, FLAGS},
     {NULL}
 };