time: use _mkgmtime and timegm to calculate unix time

vlib/time/time_nix.v

@@ -13,19 +13,8 @@ struct C.tm {
 	tm_gmtoff int // seconds
 }
 
-pub fn convert_ctime(t tm) Time {
-	return Time{
-		year: t.tm_year + 1900
-		month: t.tm_mon + 1
-		day: t.tm_mday
-		hour: t.tm_hour
-		minute: t.tm_min
-		second: t.tm_sec
-		unix: make_unix_time(t)
-	}
-}
+fn C.timegm(&tm) time_t
 
-[inline]
 fn make_unix_time(t tm) int {
-	return C.mktime(&t) + t.tm_gmtoff
+	return C.timegm(&t) as int
 }