avformat/movenc: Fix printf type

Found-by: Timothy Gu <timothygu99@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/movenc.c

@@ -1346,7 +1346,7 @@ static int mov_write_source_reference_tag(AVIOContext *pb, MOVTrack *track, cons
     int64_t pos = avio_tell(pb);
 
     if (str_size >= UINT16_MAX){
-        av_log(NULL, AV_LOG_ERROR, "reel_name length %llu is too large\n", str_size);
+        av_log(NULL, AV_LOG_ERROR, "reel_name length %"PRIu64" is too large\n", str_size);
         avio_wb16(pb, 0);
         return AVERROR(EINVAL);
     }