Fix another unused feature warning

Author: bjorn3

example/neon.rs

@@ -1,6 +1,6 @@
 // Most of these tests are copied from https://github.com/japaric/stdsimd/blob/0f4413d01c4f0c3ffbc5a69e9a37fbc7235b31a9/coresimd/arm/neon.rs
 
-#![feature(portable_simd)]
+#![cfg_attr(target_arch = "aarch64", feature(portable_simd))]
 
 #[cfg(target_arch = "aarch64")]
 use std::arch::aarch64::*;