crosvm: fix build with --no-default-feature
crosvm: fix build with --no-default-feature

Fix a warning that would prevent `presubmit --all` to pass.

BUG=None
TEST=cargo build --verbose --no-default-features

Change-Id: Iff80e5dd49b284cf20c2dd119d0b985317962a54
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosvm/+/3715086
Reviewed-by: Junichi Uekawa <6bd240a6ce32463e272bb69c685a8e946b33ac05@chromium.org>
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Commit-Queue: Alexandre Courbot <301cd6a3ea50a894b4b51e385c5eb12b9a4ffc0c@chromium.org>
Reviewed-by: Keiichi Watanabe <815e4e8f7c92a23424d7c3f51ead622bcedbc720@chromium.org>

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