Silence the hardware module where no runtime is loaded from disk

hardware::detect is read only by api::install_best, which exists with
the native feature; a build of a crate that takes the engine without it
(dr-denoise's own tests) warned that all of it was unused.

core/dr-inference-engine/src/lib.rs

@@ -21,6 +21,8 @@ use serde::{Deserialize, Serialize};
 
 mod api;
 mod engines;
+// Read only when a runtime is loaded from disk (`api::install_best`).
+#[cfg_attr(not(feature = "native"), allow(dead_code))]
 mod hardware;
 mod probe;
 mod session;