avcodec/allcodecs: make libdav1d the preferred AV1 decoder

It's considerably faster than libaom in most systems.

Reviewed-by: BBB
Signed-off-by: James Almer <jamrial@gmail.com>

libavcodec/allcodecs.c

@@ -680,7 +680,6 @@ extern AVCodec ff_pcm_mulaw_at_encoder;
 extern AVCodec ff_pcm_mulaw_at_decoder;
 extern AVCodec ff_qdmc_at_decoder;
 extern AVCodec ff_qdm2_at_decoder;
-extern AVCodec ff_libaom_av1_decoder;
 extern AVCodec ff_libaom_av1_encoder;
 extern AVCodec ff_libaribb24_decoder;
 extern AVCodec ff_libcelt_decoder;
@@ -738,6 +737,7 @@ extern AVCodec ff_idf_decoder;
 /* external libraries, that shouldn't be used by default if one of the
  * above is available */
 extern AVCodec ff_h263_v4l2m2m_encoder;
+extern AVCodec ff_libaom_av1_decoder;
 extern AVCodec ff_libopenh264_encoder;
 extern AVCodec ff_libopenh264_decoder;
 extern AVCodec ff_h264_amf_encoder;