lavf/webvttenc: use proper printf format for int64_t values

Signed-off-by: Paul B Mahol <onemda@gmail.com>

libavformat/webvttenc.c

@@ -38,9 +38,9 @@ static void webvtt_write_time(AVIOContext *pb, int64_t millisec)
     min -= 60 * hour;
 
     if (hour > 0)
-        avio_printf(pb, "%ld:", hour);
+        avio_printf(pb, "%"PRId64":", hour);
 
-    avio_printf(pb, "%02ld:%02ld.%03ld", min, sec, millisec);
+    avio_printf(pb, "%02"PRId64":%02"PRId64".%03"PRId64"", min, sec, millisec);
 }
 
 static int webvtt_write_header(AVFormatContext *ctx)