avcodec/zmbv: Mark decoder as init-threadsafe

Initializing zlib in the way we do here is threadsafe, see
https://www.zlib.net/zlib_faq.html#faq21

Reviewed-by: Tomas Härdin <tjoppen@acc.umu.se>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>

libavcodec/zmbv.c

@@ -664,5 +664,5 @@ const AVCodec ff_zmbv_decoder = {
     .close          = decode_end,
     .decode         = decode_frame,
     .capabilities   = AV_CODEC_CAP_DR1,
-    .caps_internal  = FF_CODEC_CAP_INIT_CLEANUP,
+    .caps_internal  = FF_CODEC_CAP_INIT_THREADSAFE | FF_CODEC_CAP_INIT_CLEANUP,
 };