Add an explicit allow for code covered by proofs

This code shoud already be safe because of debug-mode assertions and
formal proofs done ahead of time.

Signed-off-by: Jonathan Browne <[email protected]>

Author: JBYoshi

src/vmm/src/rate_limiter/mod.rs

@@ -116,6 +116,7 @@ impl TokenBucket {
     }
 
     // Replenishes token bucket based on elapsed time. Should only be called internally by `Self`.
+    #[allow(clippy::cast_possible_truncation)]
     fn auto_replenish(&mut self) {
         // Compute time passed since last refill/update.
         let now = Instant::now();