Fix unused import warning

An import was only used behind a feature gate.  The feature gate has
been added to the use statement to remove the warning.

bitcoin/src/sign_message.rs

@@ -6,6 +6,7 @@
 //! library is used with the `secp-recovery` feature.
 
 use hashes::{sha256d, HashEngine};
+#[cfg(feature = "secp-recovery")]
 use secp256k1::SecretKey;
 
 use crate::consensus::encode::WriteExt;