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/mpc7.c

@@ -288,7 +288,7 @@ static void mpc7_decode_flush(AVCodecContext *avctx)
     c->frames_to_skip = 32;
 }
 
-AVCodec mpc7_decoder = {
+AVCodec ff_mpc7_decoder = {
     "mpc7",
     AVMEDIA_TYPE_AUDIO,
     CODEC_ID_MUSEPACK7,