avcodec/zmbvenc: Mark encoder 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/zmbvenc.c

@@ -442,5 +442,5 @@ const AVCodec ff_zmbv_encoder = {
 #endif //ZMBV_ENABLE_24BPP
                                                      AV_PIX_FMT_BGR0,
                                                      AV_PIX_FMT_NONE },
-    .caps_internal  = FF_CODEC_CAP_INIT_CLEANUP,
+    .caps_internal  = FF_CODEC_CAP_INIT_THREADSAFE | FF_CODEC_CAP_INIT_CLEANUP,
 };