build: Add "fuzzing" as a valid cfg(..) attribute

The compiler is now able to warn if an invalid attribute (e.g like a
feature) is not available.

See https://blog.rust-lang.org/2024/05/06/check-cfg.html for more
details.

Add build.rs files in the crates that use #cfg(fuzzing) to add fuzzing
to the list of valid cfg attributes.

Signed-off-by: Rob Bradford <rbradford@rivosinc.com>

vmm/build.rs

@@ -0,0 +1,8 @@
+// Copyright © 2024 Intel Corporation
+//
+// SPDX-License-Identifier: Apache-2.0
+//
+
+fn main() {
+    println!("cargo::rustc-check-cfg=cfg(fuzzing)");
+}