Add Boost_INCLUDE_DIR to kactivities-stats autotests.

Summary:
The autotests/CMakeLists.txt doesn't add Boost_INCLUDE_DIR to the
include search path (src/CMakeLists.txt does) so the autotests fail
to build with missing Boost includes.

Reviewers: Frameworks, ivan, KActivities

Subscribers: Frameworks

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D4248