zlib-rs: properly guard AVX512 tests

these require the "std" feature being enabled (else the CPU feature detection
fails).

Signed-off-by: Fabian Grünbichler <f.gruenbichler@proxmox.com>

Author: Fabian-Gruenbichler

zlib-rs/src/adler32/avx2.rs

@@ -146,6 +146,7 @@ unsafe fn helper_32_bytes(mut adler0: u32, mut adler1: u32, src: &[__m256i]) ->
 
 #[cfg(test)]
 #[cfg(target_feature = "avx2")]
+#[cfg(feature = "std")]
 mod test {
     use super::*;