net: fix build error on master

[taiki-e]

This fixes CI failure on master: https://github.com/tokio-rs/tokio/actions/runs/1639606619

TcpSocket::linger has been added in #4324 (2 weeks ago), but #4270 was last edited a month ago, so missed this change.

error[E0599]: no method named `get_linger` found for struct `Socket` in the current scope
   --> tokio/src/net/tcp/socket.rs:381:20
    |
381 |         self.inner.get_linger()
    |                    ^^^^^^^^^^ help: there is an associated function with a similar name: `set_linger`