ALFREDAPI-532: Fix :apix-interface:javadoc

[WimCrols]

Fixes https://xenitsupport.jira.com/browse/ALFREDAPI-532

• Is CHANGELOG.md extended?
• Does this PR avoid breaking the API?
  Breaking changes include adding, changing or removing endpoints and/or JSON objects used in requests and responses.
• Does the PR comply to REST HTTP result codes policy outlined in the user guide?
• Is error handling done through a method annotated with @ExceptionHandler in the webscript classes?
• Does the PR follow our coding styleguide and other active procedures?
• Is usage of this. prefix avoided?

See README.md for full explanation.

[Brobrechts]

As discussed;

Approved, newer version of gradle (and java?) allow this code (generation of javadoc) to work without the workaround