Add support for OpenBSD

[realchonk]

This makes pprof-rs compile, but the generated flamegraphs are total garbage (at least for fuse-ufs).

[ti-chi-bot]

Welcome @realchonk! It looks like this is your first PR to tikv/pprof-rs 🎉

[ti-chi-bot]

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• 60bc3b4 add OpenBSD support

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[YangKeao]

Thanks for your contribution 🍻 .

Please sign-off your commit: you can use git commit -s --amend and git push -f to sign off your previous commit.

[YangKeao]

but the generated flamegraphs are total garbage

Let me find an OpenBSD and try to locate the issue 🤦 .