crosvm: silence unused code warning for NonZeroU8
crosvm: silence unused code warning for NonZeroU8

After a recent refactoring of display support, this import is only used
when building with the "gpu" feature enabled. Put it behind a cfg check
to avoid a warning when building without gpu support.

BUG=None
TEST=cargo build
TEST=emerge-nami crosvm

Change-Id: I4e407e09daa93c74203f3472dad5a3713b99d122
Signed-off-by: Daniel Verkamp <72bc170b46ec491f7bdd4359a1c0bfed274de40c@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosvm/+/1762448
Reviewed-by: Zach Reizner <3374927928a53d04457674c546bd4de57c2044ec@chromium.org>
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>

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