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/cook.c

@@ -1283,7 +1283,7 @@ static av_cold int cook_decode_init(AVCodecContext *avctx)
 }
 
 
-AVCodec cook_decoder =
+AVCodec ff_cook_decoder =
 {
     .name = "cook",
     .type = AVMEDIA_TYPE_AUDIO,