Define SO_INCOMING_CPU for i686-unknown-linux-gnu

[Heliozoa]

Hi! Following the example from #2119, I've tried adding the constant to i686-unknown-linux-gnu as well to fix a similar issue.

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @JohnTitor (or someone else) soon.

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[JohnTitor]

Thanks!
@bors r+

[bors]

📌 Commit 091a3dc has been approved by JohnTitor

[bors]

⌛ Testing commit 091a3dc with merge 1627d51...

[bors]

☀️ Test successful - checks-actions, checks-cirrus-freebsd-11, checks-cirrus-freebsd-12, checks-cirrus-freebsd-13
Approved by: JohnTitor
Pushing 1627d51 to master...