netdog: add attribute to ignore compiler dead-code warning

[etungsten]

Issue number:
N/A

Description of changes:
Compiling netdog with rust v1.58.1 gives a compiler warning about dead_code in netdog.

Author: Erikson Tung <etung@amazon.com>
Date:   Wed Feb 9 15:11:25 2022 -0800

    netdog: add attribute to ignore compiler dead-code warning
    
    We want to store fields from a DHCP lease, even if we don't necessary
    need to use all the fields right now.

Testing done:
Before the change, when we compile:

warning: field is never read: `dns_domain`
  --> api/netdog/src/main.rs:57:5
   |
57 |     dns_domain: Option<String>,
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = note: `#[warn(dead_code)]` on by default

warning: `netdog` (bin "netdog") generated 1 warning

After the change:
No more compiler warnings in netdog.

Terms of contribution:

By submitting this pull request, I agree that this contribution is dual-licensed under the terms of both the Apache License, version 2.0, and the MIT license.

[zmrow]

I think we probably still want to keep this field - lgtm!

🦺