Use complete include subpath.

Summary:
This makes it easier to use the header from external plugins that are
not within the kdevplatform codebase and hence do not have interfaces/*
in their include paths.

Reviewers: KDevelop, kfunk

Reviewed By: KDevelop, kfunk

Subscribers: kdevelop-devel

Projects: KDevelop

Differential Revision: https://phabricator.kde.org/D1404