TestBigEndian: Include CheckTypeSize outside macro
When using https://github.com/cristianadam/cmake-checks-cache I have noticed that CheckTypeSize would in certain cases have an empty `__check_type_size_dir` variable. The errors would point to `TestBigEndian`. By moving `include(CheckTypeSize)` outside the macro, the errors go away. Including dependencies of a module when the module is first included is simpler and cleaner anyway.