Disable unknown properties on mapping composition annotations

[thunderhook]

I'v just noticed that the plugin shows errors on annotations used for mapping composition. We should disable this, as it makes no sense.

@Retention(RetentionPolicy.CLASS)
@Mapping(target = "id", ignore = true)
@Mapping(target = "creationDate", expression = "java(new java.util.Date())")
@Mapping(target = "name", source = "groupName")
public @interface ToEntity { }