Headers missing when building loader is turned off

[pfontain]

If the tests are built with building of the loader off, the openxr headers are never included and the build fails. This is because the header dependency is only made explicit in the loader build script. A possible solution is to make that dependency explicit for all tests: https://github.com/KhronosGroup/OpenXR-SDK-Source/pull/439

This was detected while using the latest release with the Quest SDK: https://github.com/pfontain/OpenXR-SDK-Source/commit/2e7def6b72a98c293ff1b4d8d7e767060f60bdc3