Add: Mexican Peso currency

OpenTTD doesn't have any pesos, so I've added Mexican pesos,
at its current exchange rate: 1GBP = 24.39MXN, rounded to 24.

src/currency.h

@@ -59,6 +59,7 @@ enum Currencies {
 	CURRENCY_GEL,       ///< Georgian Lari
 	CURRENCY_IRR,       ///< Iranian Rial
 	CURRENCY_RUB,       ///< New Russian Ruble
+	CURRENCY_MXN,       ///< Mexican Peso
 	CURRENCY_END,       ///< always the last item
 };