avcodec/mmvideo: Mark decoder as init-threadsafe

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>

libavcodec/mmvideo.c

@@ -248,4 +248,5 @@ const AVCodec ff_mmvideo_decoder = {
     .close          = mm_decode_end,
     .decode         = mm_decode_frame,
     .capabilities   = AV_CODEC_CAP_DR1,
+    .caps_internal  = FF_CODEC_CAP_INIT_THREADSAFE,
 };