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/chomp_bsf.c

@@ -40,7 +40,7 @@ static int chomp_filter(AVBitStreamFilterContext *bsfc,
 /**
  * This filter removes a string of NULL bytes from the end of a packet.
  */
-AVBitStreamFilter chomp_bsf = {
+AVBitStreamFilter ff_chomp_bsf = {
     "chomp",
     0,
     chomp_filter,