Make use rand::... gated on cfg(feature = "rand")

This is no longer actively breaking our no_std build, but I think
it's still technically a minor bug, and further case of issue dalek-cryptography#108

src/secret.rs

@@ -17,6 +17,7 @@ use curve25519_dalek::digest::Digest;
 use curve25519_dalek::edwards::CompressedEdwardsY;
 use curve25519_dalek::scalar::Scalar;
 
+#[cfg(feature = "rand")]
 use rand::{CryptoRng, RngCore};
 
 use sha2::Sha512;