Fix std builds when serde is enabled.

Cargo.toml

@@ -50,7 +50,7 @@ harness = false
 
 [features]
 default = ["std", "rand", "u64_backend"]
-std = ["curve25519-dalek/std", "ed25519/std", "serde_crate/std", "sha2/std", "rand/std"]
+std = ["curve25519-dalek/std", "ed25519/std", "serde_bytes/std", "serde_crate/std", "sha2/std", "rand/std"]
 alloc = ["curve25519-dalek/alloc", "rand/alloc", "serde_bytes/alloc", "zeroize/alloc"]
 nightly = ["curve25519-dalek/nightly"]
 serde = ["serde_crate", "serde_bytes", "ed25519/serde"]