refactor(identity): fix import warnings in keypair.rs

- Adding conditional `cfg` in `keypair.rs` imports.
- Adding `#[allow(unused_variables)]` on the top of the `sign` function.

Pull-Request: #4047.

identity/src/keypair.rs

@@ -18,6 +18,12 @@
 // FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
 // DEALINGS IN THE SOFTWARE.
 
+#[cfg(any(
+    feature = "ecdsa",
+    feature = "secp256k1",
+    feature = "ed25519",
+    feature = "rsa"
+))]
 use crate::error::OtherVariantError;
 use crate::error::{DecodingError, SigningError};
 #[cfg(any(
@@ -27,7 +33,19 @@ use crate::error::{DecodingError, SigningError};
     feature = "rsa"
 ))]
 use crate::proto;
+#[cfg(any(
+    feature = "ecdsa",
+    feature = "secp256k1",
+    feature = "ed25519",
+    feature = "rsa"
+))]
 use quick_protobuf::{BytesReader, Writer};
+#[cfg(any(
+    feature = "ecdsa",
+    feature = "secp256k1",
+    feature = "ed25519",
+    feature = "rsa"
+))]
 use std::convert::TryFrom;
 
 #[cfg(feature = "ed25519")]
@@ -159,6 +177,7 @@ impl Keypair {
 
     /// Sign a message using the private key of this keypair, producing
     /// a signature that can be verified using the corresponding public key.
+    #[allow(unused_variables)]
     pub fn sign(&self, msg: &[u8]) -> Result<Vec<u8>, SigningError> {
         match self.keypair {
             #[cfg(feature = "ed25519")]