Akonadi: prefer binaries from the same directory

Summary:
This allows to test changes to akonadiserver without installing it
(given that all binaries are in build/bin)

Test Plan: my debug output in akonadiserver now shows up, running a test program from the builddir.

Reviewers: dvratil, vkrause

Reviewed By: dvratil

Subscribers: kde-pim

Differential Revision: https://phabricator.kde.org/D19404