Apply license header check to generator module

[gunnarmorling]

There were several files without license header in the jfrunit-events-generator module, which should have failed the build, but it didn't. Let's add the License Maven plug-in, as it already should be the case for the jfrunit-core module.

[dve]

Would it not be better to place it in the aggreagtor/parent pom not not duplicate the configuration?

[dve]

Ok, I see it has been already done - so I think this can be closed