sys_util: Fix unused trait warning
sys_util: Fix unused trait warning

Commit "d5aa99542: disk: use seek to determine file length" added these
traits to sys_util but didn't use them.

Change-Id: I0babbbc75450e20f9f7ae7d5f94dd6285c12a8bb
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosvm/+/1999948
Tested-by: Dylan Reid <c791bfe44996c1eed9c7149a0c5f12a86d58eb85@chromium.org>
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Auto-Submit: Dylan Reid <c791bfe44996c1eed9c7149a0c5f12a86d58eb85@chromium.org>
Commit-Queue: Zach Reizner <3374927928a53d04457674c546bd4de57c2044ec@chromium.org>
Reviewed-by: Zach Reizner <3374927928a53d04457674c546bd4de57c2044ec@chromium.org>

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