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>
(cherry picked from commit d36beb3f69)

libavcodec/alsdec.c

@@ -1737,7 +1737,7 @@ static av_cold void flush(AVCodecContext *avctx)
 }
 
 
-AVCodec als_decoder = {
+AVCodec ff_als_decoder = {
     "als",
     AVMEDIA_TYPE_AUDIO,
     CODEC_ID_MP4ALS,