lavf: make av_set_pts_info private.

It's supposed to be called only from (de)muxers.
diff --git a/libavformat/assdec.c b/libavformat/assdec.c
index 6b9c3de..b100f2c 100644
--- a/libavformat/assdec.c
+++ b/libavformat/assdec.c
@@ -86,7 +86,7 @@
     st = avformat_new_stream(s, NULL);
     if (!st)
         return -1;
-    av_set_pts_info(st, 64, 1, 100);
+    avpriv_set_pts_info(st, 64, 1, 100);
     st->codec->codec_type = AVMEDIA_TYPE_SUBTITLE;
     st->codec->codec_id= CODEC_ID_SSA;