Force using a 64bit intermediate when calculating calendar time with av_timegm().

Fixes a fate failure with icc 13.1

libavutil/parseutils.c

@@ -519,7 +519,7 @@ time_t av_timegm(struct tm *tm)
         y--;
     }
 
-    t = 86400 *
+    t = 86400LL *
         (d + (153 * m - 457) / 5 + 365 * y + y / 4 - y / 100 + y / 400 - 719469);
 
     t += 3600 * tm->tm_hour + 60 * tm->tm_min + tm->tm_sec;