Enable the subtle/const-generics feature

`subtle` is an optional dependency, but when it's enabled, this also
enables its `const-generics` feature

Author: tarcieri

Cargo.toml

@@ -22,7 +22,7 @@ typenum = { version = "1.17", features = ["const-generics"] }
 # optional dependencies
 bytemuck = { version = "1", optional = true, default-features = false }
 serde = { version = "1", optional = true, default-features = false }
-subtle = { version = "2", optional = true, default-features = false }
+subtle = { version = "2", optional = true, default-features = false, features = ["const-generics"] }
 zeroize = { version = "1.8", optional = true, default-features = false }
 
 [dev-dependencies]