deactivating dns-feature doesn't compile

[gnunicorn]

In substrate we are (effectively) building libp2p (0.34.0 but I can also reproduce it on latest master) for sc-network as/with

cargo build --no-default-features --features "identify,kad,mdns,mplex,noise,ping,request-response,tcp-async-io,websocket,yamux"

Which fails, dns is not activated but used by build_tcp_ws_noise_mplex_yamux:

   Compiling libp2p v0.35.0 (/home/ben/Dev/parity/rust-libp2p)
error[E0433]: failed to resolve: use of undeclared crate or module `dns`
   --> src/lib.rs:293:25
    |
293 |         let transport = dns::DnsConfig::new(tcp)?;
    |                         ^^^ use of undeclared crate or module `dns`

error: aborting due to previous error

This probably didn't show up earlier, as we rarely build that package separately and in node/cli build it with default-features enabled. Adding dns to the feature list and all goes fine.

[romanb]

I think this has been addressed along the way in #1927, which can hopefully be merged and released soon. Of course, if this issue has any urgency, we could patch it separately before #1927 is merged.

[gnunicorn]

no urgency. the now here linked patch shows that it actually originated from a wrong specification of features in substrate, so adding dns there was all that's needed for us to continue. The issues state here is still correct though.