Fix postcode_iso3166_alpha2_field validation (#1359)

Fixed an issue where the post codes regexes would not get initialised
for postcode_iso3166_alpha2_field validation.

## Fixes Or Enhances


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- [x] Tests exist or have been written that cover this particular
change.

@go-playground/validator-maintainers

Author: ddevcap

baked_in.go

@@ -1417,6 +1417,7 @@ func isPostcodeByIso3166Alpha2Field(fl FieldLevel) bool {
 		panic(fmt.Sprintf("Bad field type %T", currentField.Interface()))
 	}
 
+	postcodeRegexInit.Do(initPostcodes)
 	reg, found := postCodeRegexDict[currentField.String()]
 	if !found {
 		return false