MSVC warning C4459: declaration of 'cleanup' hides global declaration

When using libunifex with MSVC 19.34.31935 and level 4 warnings enabled, I'm seeing this warning:

[spawn_future.hpp(736,29)][1]: warning [C4459][2]: declaration of 'cleanup' hides global declaration
[stream_concepts.hpp(77,5)][3]: note: see declaration of 'unifex::_streams::cleanup'

If we really want the stream `cleanup` to be a global declaration we should avoid using the name `cleanup` elsewhere.

[1]: https://github.com/facebookexperimental/libunifex/blob/main/include/unifex/spawn_future.hpp#L736
[2]: https://learn.microsoft.com/en-us/cpp/error-messages/compiler-warnings/compiler-warning-level-4-c4459?view=msvc-170
[3]: https://github.com/facebookexperimental/libunifex/blob/main/include/unifex/stream_concepts.hpp#L77