Adds keyword "default" to the API-linter.

This will fix issue when linting Java 8 default methods on interfaces,
e.g. Ie46a876dcb14b46b8b2584735c106c25655ec6cf.

Test: Manualy tried that the linter no longer gives the error.

Change-Id: I68ade6ba6401439667408076a5454caef77ef72d