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/sgienc.c

@@ -159,7 +159,7 @@ static int encode_frame(AVCodecContext *avctx, unsigned char *buf,
     return buf - orig_buf;
 }
 
-AVCodec sgi_encoder = {
+AVCodec ff_sgi_encoder = {
     "sgi",
     AVMEDIA_TYPE_VIDEO,
     CODEC_ID_SGI,