avcodec/yuv4dec: Mark decoder as init-threadsafe

Reviewed-by: Anton Khirnov <anton@khirnov.net>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>

libavcodec/yuv4dec.c

@@ -81,4 +81,5 @@ AVCodec ff_yuv4_decoder = {
     .init         = yuv4_decode_init,
     .decode       = yuv4_decode_frame,
     .capabilities = AV_CODEC_CAP_DR1,
+    .caps_internal = FF_CODEC_CAP_INIT_THREADSAFE,
 };