mpegts: fix a unused function warning

note the function is used under #if
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/mpegts.c

@@ -2107,7 +2107,7 @@ static int mpegts_read_close(AVFormatContext *s)
     return 0;
 }
 
-static int64_t mpegts_get_pcr(AVFormatContext *s, int stream_index,
+static av_unused int64_t mpegts_get_pcr(AVFormatContext *s, int stream_index,
                               int64_t *ppos, int64_t pos_limit)
 {
     MpegTSContext *ts = s->priv_data;