Suppress some dead code warnings for DnsResolver

error: field `headers` is never read
   --> packages/modules/DnsResolver/doh/connection/driver.rs:110:9
    |
108 | pub struct Stream {
    |            ------ field in this struct
109 |     /// Response headers
110 |     pub headers: Vec<h3::Header>,
    |         ^^^^^^^
    |
    = note: `Stream` has a derived impl for the trait `Debug`, but this
is intentionally ignored during dead code analysis
    = note: `-D dead-code` implied by `-D warnings`
    = help: to override `-D warnings` add `#[allow(dead_code)]`

error: field `0` is never read
  --> packages/modules/DnsResolver/doh/network/driver.rs:63:12
   |
63 |     Failed(Arc<anyhow::Error>),
   |     ------ ^^^^^^^^^^^^^^^^^^
   |     |
   |     field in this variant
   |
help: consider changing the field to be of unit type to suppress this
warning while preserving the field numbering, or remove the field
   |
63 |     Failed(()),
   |            ~~

Bug: http://b/346588808
Test: ./toolchain/android_rust/tools/test_compiler.py --prebuilt-path dist/rust-linux.tar.xz --target aosp_cf_x86_64_phone --image --reuse-prebuilt
Change-Id: I4cdc0503b143b0eba914b1164a1db8acf3904457