add setns posix namespace declaration

[Laeeth]

No description provided.

[dlang-bot]

Thanks for your pull request and interest in making D better, @Laeeth! We are looking forward to reviewing it, and you should be hearing from a maintainer soon.
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Testing this PR locally

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[RazvanN7]

Hi Laeeth! Can you please rebase this PR? I tried rebasing for you, but it seems that I don't have enough rights to push to your fork. Thanks!