Don't export internal classes when not building autotests.

Summary: surely this is the reason for having a different export macro

Test Plan:
with BUILD_TESTING=FALSE, the generated header just says
#define AKONADI_TESTS_EXPORT, and everything still builds.

Reviewers: dvratil

Reviewed By: dvratil

Subscribers: KDE PIM

Tags: KDE PIM

Differential Revision: https://phabricator.kde.org/D4694