[rust-server] Fix server example on non-Linux

[mattdowdell]

The server example is broken because it tries to import use openssl::ssl::SslAcceptorBuilder; unconditionally when it won't be present on MacOS, Windows or iOS and then conditionally provides create causing compile errors on the aforementioned platforms which try to unconditionally import said function in main.rs.

To fix, we simply move the condition from the function to the import and all is happy again :)

To verify, generate the code from any spec, then attempt to use the server example on one of the problem platforms.

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cc @frol @farcaller @richardwhiuk @paladinzh for review.

[mattdowdell]

• Unsure why CircleCI build is failing because they don't allow anonymous access and I didn't really want to grant access to all my repos.
• Appveyor build is failing due to some C# issues which I don't see could be caused by this. Given when other PRs started failing, I'd guess [C#][netcore] better support for additional properties in the payload #7396 is the cause.

[wing328]

Thanks for the PR. I've merged the latest master into this branch. Let's see if that fixes the CI failures.

[richardwhiuk]

All looks good to me.

Thanks!