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.