io_queue: properly declare io_queue_for_tests
io_queue: properly declare io_queue_for_tests

Fixes the following error with clang++ --std=c++20
```
../../include/seastar/core/io_queue.hh:170:12: error: class 'io_queue_for_tests' was previously declared as a struct; this is valid, but may result in linker errors under the Microsoft C++ ABI [-Werror,-Wmismatched-tags]
    friend class ::io_queue_for_tests;
           ^
../../include/seastar/core/io_queue.hh:33:8: note: previous use is here
struct io_queue_for_tests;
       ^

Signed-off-by: Benny Halevy <23c997c90eb8537635e9392d48e7af1bf1b2ca22@scylladb.com>
Message-Id: <20220126080637.974944-1-23c997c90eb8537635e9392d48e7af1bf1b2ca22@scylladb.com>

syuu1228/seastar,syuu1228/seastar,scylladb/seastar,avikivity/seastar,scylladb/seastar,syuu1228/seastar,scylladb/seastar,avikivity/seastar,avikivity/seastar