Fix clippy lint (#90)

Author: hkratz

src/lib.rs

@@ -14,7 +14,10 @@
 #![deny(missing_docs)]
 #![cfg_attr(not(feature = "std"), no_std)]
 #![cfg_attr(docsrs, feature(doc_cfg))]
-#![cfg_attr(feature = "aarch64_neon_prefetch", feature(stdarch_aarch64_prefetch))]
+#![cfg_attr(
+    all(target_arch = "aarch64", feature = "aarch64_neon_prefetch"),
+    feature(stdarch_aarch64_prefetch)
+)]
 
 //! Blazingly fast API-compatible UTF-8 validation for Rust using SIMD extensions, based on the implementation from
 //! [simdjson](https://github.com/simdjson/simdjson). Originally ported to Rust by the developers of [simd-json.rs](https://simd-json.rs), but now heavily improved.