avcodec/8bps: Mark decoder as init-threadsafe

The only unorthodox thing that this codec's init function does
is calling ff_get_format(). Yet this is supposed to be save,
as any get_format callback already has to deal with the scenario
of different AVCodecContext's calling it simultaneously.
Reviewed-by: Anton Khirnov <anton@khirnov.net>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>

libavcodec/8bps.c

@@ -182,4 +182,5 @@ const AVCodec ff_eightbps_decoder = {
     .init           = decode_init,
     .decode         = decode_frame,
     .capabilities   = AV_CODEC_CAP_DR1,
+    .caps_internal  = FF_CODEC_CAP_INIT_THREADSAFE,
 };