avcodec/svq3: Mark decoder as init-threadsafe

The only interesting thing done in SVQ3's init function
is using zlib, but this is fine: https://zlib.net/zlib_faq.html#faq21

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

libavcodec/svq3.c

@@ -1601,5 +1601,5 @@ const AVCodec ff_svq3_decoder = {
                       AV_CODEC_CAP_DELAY,
     .pix_fmts       = (const enum AVPixelFormat[]) { AV_PIX_FMT_YUVJ420P,
                                                      AV_PIX_FMT_NONE},
-    .caps_internal  = FF_CODEC_CAP_INIT_CLEANUP,
+    .caps_internal  = FF_CODEC_CAP_INIT_THREADSAFE | FF_CODEC_CAP_INIT_CLEANUP,
 };