Port ClangClassHelper to also fill the include_files var

Summary:
The "included_files" variable should contain the include directive arguments
for the base classes of the created class.

Reviewers: KDevelop, mwolff, kfunk

Reviewed By: KDevelop, kfunk

Subscribers: kfunk, kdevelop-devel

Differential Revision: https://phabricator.kde.org/D4430