avcodec/vorbisdec: Mark decoder as init-threadsafe

It does not modify any static data in its init function.
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>

libavcodec/vorbisdec.c

@@ -1891,7 +1891,7 @@ const AVCodec ff_vorbis_decoder = {
     .decode          = vorbis_decode_frame,
     .flush           = vorbis_decode_flush,
     .capabilities    = AV_CODEC_CAP_DR1 | AV_CODEC_CAP_CHANNEL_CONF,
-    .caps_internal   = FF_CODEC_CAP_INIT_CLEANUP,
+    .caps_internal   = FF_CODEC_CAP_INIT_THREADSAFE | FF_CODEC_CAP_INIT_CLEANUP,
     .channel_layouts = ff_vorbis_channel_layouts,
     .sample_fmts     = (const enum AVSampleFormat[]) { AV_SAMPLE_FMT_FLTP,
                                                        AV_SAMPLE_FMT_NONE },