Add field `private` to Cargo.toml

[JanLikar]

I Accidentally closed #2218, this is the new PR.

[Fix #2202]

[rust-highfive]

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

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[JanLikar]

r? @alexcrichton

[alexcrichton]

@bors: r+ d28f8bb

[bors]

⌛️ Testing commit d28f8bb with merge 6ffd134...

[bors]

☀️ Test successful - cargo-linux-32, cargo-linux-64, cargo-mac-32, cargo-mac-64, cargo-win-gnu-32, cargo-win-gnu-64, cargo-win-msvc-32, cargo-win-msvc-64