Fixed `Subprocess::ChildHook::CHDIR()` to use `os::chdir()`.
Fixed `Subprocess::ChildHook::CHDIR()` to use `os::chdir()`.

This needed to use the Stout API so that the correct Windows
implementation is used, as `::chdir` is part of the CRT.

Also included used but not included `stout/os/*` headers.

Review: https://reviews.apache.org/r/66443

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