avoid compiling avx512_detected() when the "c_avx512" feature is disa…

…bled

rust-lang/rust#68905 is currently causing
nightly builds to fail, unless `--no-default-features` is used. This
change means that the default build will succeed, and the failure will
only happen when the "c_avx512" is enabled. The `b3sum` crate will still
fail to build on nightly, because it enables that feature, but most
callers should start succeeding on nightly.

src/platform.rs

@@ -252,6 +252,7 @@ impl Platform {
 
 // Note that AVX-512 is divided into multiple featuresets, and we use two of
 // them, F and VL.
+#[cfg(feature = "c_avx512")]
 #[cfg(any(target_arch = "x86", target_arch = "x86_64"))]
 #[inline(always)]
 pub fn avx512_detected() -> bool {