Add ff_ prefix to data symbols of encoders, decoders, hwaccel, parsers, bsf.

None of these symbols should be accessed directly, so declare them as
hidden.

Signed-off-by: Mans Rullgard <mans@mansr.com>

libavcodec/libvpxdec.c

@@ -111,7 +111,7 @@ static av_cold int vp8_free(AVCodecContext *avctx)
     return 0;
 }
 
-AVCodec libvpx_decoder = {
+AVCodec ff_libvpx_decoder = {
     "libvpx",
     AVMEDIA_TYPE_VIDEO,
     CODEC_ID_VP8,