Unreviewed. Fix GTK+ clean build after r175930.

Partial rollout of r175930 to not build the new unit test that
broke clean builds in GTK+ port.

* TestWebKitAPI/PlatformGTK.cmake:

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@176015 268f45cc-cd09-0410-ab3c-d52691b4dbfc