Apply suggestions from code review

Co-authored-by: Jean-Philippe Raynaud <jp.raynaud@gmail.com>

Author: curiecrypt

mithril-stm/src/lib.rs

@@ -124,7 +124,7 @@ pub use signature_scheme::{
     BlsVerificationKeyProofOfPossession,
 };
 
-#[cfg(feature = "future_snark")]
+#[cfg(all(feature = "benchmark-internals", feature = "future_snark"))]
 pub use signature_scheme::{SchnorrSignature, SchnorrSigningKey, SchnorrVerificationKey};
 
 /// The quantity of stake held by a party, represented as a `u64`.