Update to use feature flag for simd

Signed-off-by: Michael Lodder <redmike7@gmail.com>

curve25519-dalek/Cargo.toml

@@ -4,7 +4,7 @@ name = "curve25519-dalek-ml"
 # - update CHANGELOG
 # - update README if required by semver
 # - if README was updated, also update module documentation in src/lib.rs
-version = "4.2.0"
+version = "4.2.1"
 edition = "2021"
 authors = ["Isis Lovecruft <isis@patternsinthevoid.net>",
            "Henry de Valence <hdevalence@hdevalence.ca>",