Add the missing license header reminded by Cédric
Add the missing license header reminded by Cédric

https://github.com/apache/groovy/pull/1488#discussion_r576291673

apache/incubator-groovy,apache/groovy,apache/incubator-groovy,apache/incubator-groovy,paulk-asert/groovy,apache/groovy,apache/groovy,paulk-asert/groovy,apache/groovy,paulk-asert/groovy,paulk-asert/groovy,apache/incubator-groovy