rust: Add deny(unused_must_use)

We're maintaining the operating system, let's hard require
that errors are checked.

rust/src/lib.rs

@@ -4,6 +4,8 @@
  * SPDX-License-Identifier: Apache-2.0 OR MIT
  */
 
+#![deny(unused_must_use)]
+
 // pub(crate) utilities
 mod ffiutil;
 mod includes;