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/ac3enc_float.c

@@ -109,7 +109,7 @@ static void scale_coefficients(AC3EncodeContext *s)
 }
 
 
-AVCodec ac3_encoder = {
+AVCodec ff_ac3_encoder = {
     "ac3",
     AVMEDIA_TYPE_AUDIO,
     CODEC_ID_AC3,