tests: fix macos build failure

Summary: Add boost include similiar to autotests/CMakeLists.txt

Test Plan:
make && make tests
bin/KActivitiesStatsTestApp.app/Contents/MacOS/KActivitiesStatsTestApp

Reviewers: cullmann

Reviewed By: cullmann

Subscribers: cullmann, kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D24811