the derive.rs tests also depends on the digest feature (#1339)

signature/tests/derive.rs

@@ -1,6 +1,6 @@
 //! Tests for code generated by `signature_derive`
 
-#![cfg(feature = "derive")]
+#![cfg(all(feature = "derive", feature = "digest"))]
 
 use digest::{generic_array::GenericArray, Digest, OutputSizeUser};
 use hex_literal::hex;