setup: Suppress warning on fields that are only used by TPM
setup: Suppress warning on fields that are only used by TPM

Warnings when building without `--features tpm` previously:

    warning: field is never used: `uid`
       --> src/linux.rs:696:5
        |
    696 |     uid: uid_t,
        |     ^^^^^^^^^^
        |

    warning: field is never used: `gid`
       --> src/linux.rs:697:5
        |
    697 |     gid: gid_t,
        |     ^^^^^^^^^^

TEST=cargo check
TEST=cargo check --features tpm

Change-Id: Id6beee4237e70d9107b36601583f599f85e5c74d
Reviewed-on: https://chromium-review.googlesource.com/1500874
Commit-Ready: ChromeOS CL Exonerator Bot <fa429e5907f00867f797423aea74327bb02c91b6@appspot.gserviceaccount.com>
Tested-by: David Tolnay <1480464b87745a0822a0e8b7545ca981d8e7352e@chromium.org>
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Zach Reizner <3374927928a53d04457674c546bd4de57c2044ec@chromium.org>
Reviewed-by: Daniel Verkamp <72bc170b46ec491f7bdd4359a1c0bfed274de40c@chromium.org>

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