asf: Use time_t where needed

gmtime takes a time_t not an uint64_t.

libavformat/asfdec.c

@@ -641,7 +641,7 @@ static int asf_read_properties(AVFormatContext *s, const GUIDParseTable *g)
 {
     ASFContext *asf = s->priv_data;
     AVIOContext *pb = s->pb;
-    uint64_t creation_time;
+    time_t creation_time;
 
     avio_rl64(pb); // read object size
     avio_skip(pb, 16); // skip File ID