pack drkonqi into a transient static lib we can use for unit testing

Summary:
without the static lib this is fairly cumbersome because most classes
depend on config-drkonqi and more importantly the qdebug category. by
having everything in a static lib we can simply pull that in as a link
target and get access to all compilation units and have include dirs
set up etc.

Test Plan: builds & tests pass

Reviewers: #plasma, apol

Reviewed By: apol

Subscribers: apol, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D22135