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 <[email protected]>
(cherry picked from commit d36beb3f6902b1217beda576aa18abf7eb72b03c)
diff --git a/libavcodec/mpc7.c b/libavcodec/mpc7.c
index 5ac2579..a77648f 100644
--- a/libavcodec/mpc7.c
+++ b/libavcodec/mpc7.c
@@ -288,7 +288,7 @@
     c->frames_to_skip = 32;
 }
 
-AVCodec mpc7_decoder = {
+AVCodec ff_mpc7_decoder = {
     "mpc7",
     AVMEDIA_TYPE_AUDIO,
     CODEC_ID_MUSEPACK7,