avcodec/libvpxdec: Constify VP9-decoder

Possible since 8d226fb978.

Reviewed-by: James Zern <jzern-at-google.com@ffmpeg.org>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>

libavcodec/allcodecs.c

@@ -798,7 +798,7 @@ extern const FFCodec ff_libvorbis_decoder;
 extern const FFCodec ff_libvpx_vp8_encoder;
 extern const FFCodec ff_libvpx_vp8_decoder;
 extern FFCodec ff_libvpx_vp9_encoder;
-extern FFCodec ff_libvpx_vp9_decoder;
+extern const FFCodec ff_libvpx_vp9_decoder;
 /* preferred over libwebp */
 extern const FFCodec ff_libwebp_anim_encoder;
 extern const FFCodec ff_libwebp_encoder;

libavcodec/libvpxdec.c

@@ -377,7 +377,7 @@ static av_cold int vp9_init(AVCodecContext *avctx)
     return vpx_init(avctx, &ctx->decoder, vpx_codec_vp9_dx());
 }
 
-FFCodec ff_libvpx_vp9_decoder = {
+const FFCodec ff_libvpx_vp9_decoder = {
     .p.name         = "libvpx-vp9",
     CODEC_LONG_NAME("libvpx VP9"),
     .p.type         = AVMEDIA_TYPE_VIDEO,