Add MX-CST/CDT Time Zone

Add the Mexico City CST/CDT timezone.
The start and finish date for DLS differs with the US-CST/CDT rules.

wled00/ntp.cpp

@@ -31,6 +31,7 @@ Timezone* tz;
 #define TZ_HAWAII              18
 #define TZ_NOVOSIBIRSK         19
 #define TZ_ANCHORAGE           20
+#define TZ_MX_CENTRAL          21  
 #define TZ_INIT               255
 
 byte tzCurrent = TZ_INIT; //uninitialized
@@ -141,6 +142,11 @@ void updateTimezone() {
       tcrStandard = {First, Sun, Nov, 2, -540};   //AKST = UTC - 9 hours
       break;
     }
+     case TZ_MX_CENTRAL : {
+      tcrDaylight = {First, Sun, Apr, 2, -300};  //CDT = UTC - 5 hours
+      tcrStandard = {Last,  Sun, Oct, 2, -360};  //CST = UTC - 6 hours
+      break;
+    }
   }
 
   tzCurrent = currentTimezone;