Fix CurveMQ for libsodium feature

[esemeniuc]

I tried https://github.com/bartols/rust-zmq, https://github.com/SylvainMartel/rust-zmq, and https://github.com/conanoc/rust-zmq/tree/libsodium, they didn't work for me. Only 0.9.2 worked.

Simple test:

        let publisher_keypair = zmq::CurveKeyPair::new().unwrap();
        let subscriber_keypair = zmq::CurveKeyPair::new().unwrap();

        println!("Publisher Public Key: {:?}", publisher_keypair.public_key);
        println!("Publisher Secret Key: {:?}", publisher_keypair.secret_key);
        println!("Subscriber Public Key: {:?}", subscriber_keypair.public_key);
        println!("Subscriber Secret Key: {:?}", subscriber_keypair.secret_key);

Fixes: #357

References:
jean-airoldie/zeromq-src-rs#32