stdlib: remove unused warning on sock_wait() (#8303)

The function is used in 'feature = ssl'. However, the current codebase
defines it without a conditional compilation expression and always be
compiled.

This commit removes the unused warning by adding a conditional
compilation expression, '#[cfg(feature = "ssl")]'.

crates/stdlib/src/socket.rs

@@ -2444,6 +2444,7 @@ mod _socket {
     }
 
     /// returns Ok(true) on timeout
+    #[cfg(feature = "ssl")]
     pub(crate) fn sock_wait(
         sock: &Socket,
         wait_kind: SockWaitKind,