Akonadi: add missing dependencies on included XSL files

Summary:
Editing e.g. src/server/storage/entities-source.xsl
didn't lead to entities.cpp being regenerated.

Test Plan: make now rebuilds entities.cpp

Reviewers: dvratil, vkrause

Reviewed By: vkrause

Subscribers: kde-pim

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D18887