Fix clippy error in balloon.rs
Fix clippy error in balloon.rs

This snuck through as our builders were temporarily not checking
clippy in presubmit. See https://crrev.com/c/3498847

BUG=None
TEST=./tools/clippy

Change-Id: I38ca27728ab4e108539407f5499484c2b881dc47
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosvm/+/3499740
Auto-Submit: Dennis Kempin <cd09796fb571bec2782819dbfd33307f65b1c778@google.com>
Reviewed-by: Daniel Verkamp <72bc170b46ec491f7bdd4359a1c0bfed274de40c@chromium.org>
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Anton Romanov <7c8c28e1283c1d87f2c3df4d6b5c74b6ae02bcd9@google.com>
Commit-Queue: Anton Romanov <7c8c28e1283c1d87f2c3df4d6b5c74b6ae02bcd9@google.com>

google/crosvm,google/crosvm,google/crosvm,google/crosvm