avcodec/exr: Mark decoder as init-threadsafe

This decoder does not initialize any static data in its init function.
Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>

libavcodec/exr.c

@@ -2351,5 +2351,6 @@ const AVCodec ff_exr_decoder = {
     .decode           = decode_frame,
     .capabilities     = AV_CODEC_CAP_DR1 | AV_CODEC_CAP_FRAME_THREADS |
                         AV_CODEC_CAP_SLICE_THREADS,
+    .caps_internal    = FF_CODEC_CAP_INIT_THREADSAFE,
     .priv_class       = &exr_class,
 };