Fix unused import clippy lint

One import is unused with no-default-features.

Feature gate it.

Author: jamillambert

consensus_encoding/tests/encode.rs

@@ -5,7 +5,9 @@
 #[cfg(feature = "std")]
 use std::io::{Cursor, Write};
 
-use bitcoin_consensus_encoding::{ArrayEncoder, BytesEncoder, Encodable, Encoder};
+use bitcoin_consensus_encoding::{ArrayEncoder, BytesEncoder, Encoder};
+#[cfg(feature = "alloc")]
+use bitcoin_consensus_encoding::{Encodable};
 
 // Simple test type that implements Encodable.
 #[cfg(feature = "alloc")]