lavf/mxfdec: use more widely supported time format string

MSVC lacks support for %T and %F, but the standard specifies them
only as shorthands to the long variants.
Reviewed-by: Matthieu Bouron <matthieu.bouron@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/mxfdec.c

@@ -1671,7 +1671,7 @@ static int mxf_timestamp_to_str(uint64_t timestamp, char **str)
     *str = av_mallocz(32);
     if (!*str)
         return AVERROR(ENOMEM);
-    strftime(*str, 32, "%F %T", &time);
+    strftime(*str, 32, "%Y-%m-%d %H:%M:%S", &time);
 
     return 0;
 }