lavc: make avpriv_mpa_decode_header private on next bump

It's not used by anything outside of lavc anymore.

libavcodec/mpegaudio_parser.c

@@ -67,7 +67,7 @@ static int mpegaudio_parse(AVCodecParserContext *s1,
 
                 state= (state<<8) + buf[i++];
 
-                ret = avpriv_mpa_decode_header(avctx, state, &sr, &channels, &frame_size, &bit_rate);
+                ret = ff_mpa_decode_header(avctx, state, &sr, &channels, &frame_size, &bit_rate);
                 if (ret < 4) {
                     if (i > 4)
                         s->header_count = -2;

libavcodec/mpegaudiodecheader.c

@@ -118,7 +118,8 @@ int avpriv_mpegaudio_decode_header(MPADecodeHeader *s, uint32_t header)
     return 0;
 }
 
-int avpriv_mpa_decode_header(AVCodecContext *avctx, uint32_t head, int *sample_rate, int *channels, int *frame_size, int *bit_rate)
+int ff_mpa_decode_header(AVCodecContext *avctx, uint32_t head, int *sample_rate,
+                         int *channels, int *frame_size, int *bit_rate)
 {
     MPADecodeHeader s1, *s = &s1;
 
@@ -151,3 +152,10 @@ int avpriv_mpa_decode_header(AVCodecContext *avctx, uint32_t head, int *sample_r
     *bit_rate = s->bit_rate;
     return s->frame_size;
 }
+
+#if LIBAVCODEC_VERSION_MAJOR < 57
+int avpriv_mpa_decode_header(AVCodecContext *avctx, uint32_t head, int *sample_rate, int *channels, int *frame_size, int *bit_rate)
+{
+    return ff_mpa_decode_header(avctx, head, sample_rate, channels, frame_size, bit_rate);
+}
+#endif

libavcodec/mpegaudiodecheader.h

@@ -54,7 +54,12 @@ int avpriv_mpegaudio_decode_header(MPADecodeHeader *s, uint32_t header);
 
 /* useful helper to get mpeg audio stream infos. Return -1 if error in
    header, otherwise the coded frame size in bytes */
+int ff_mpa_decode_header(AVCodecContext *avctx, uint32_t head, int *sample_rate,
+                         int *channels, int *frame_size, int *bitrate);
+
+#if LIBAVCODEC_VERSION_MAJOR < 57
 int avpriv_mpa_decode_header(AVCodecContext *avctx, uint32_t head, int *sample_rate, int *channels, int *frame_size, int *bitrate);
+#endif
 
 /* fast header check for resync */
 static inline int ff_mpa_check_header(uint32_t header){