async_utils.cc was introduced in e1411f39, so let's
update the cmake building system to build it. without
which, we'd run into link failure like:
```
ld.lld: error: undefined symbol: to_mutation_gently(canonical_mutation const&, seastar::lw_shared_ptr<schema const>)
>>> referenced by storage_service.cc
>>> storage_service.cc.o:(service::storage_service::merge_topology_snapshot(service::raft_snapshot)) in archive service/Dev/libservice.a
>>> referenced by group0_state_machine.cc
>>> group0_state_machine.cc.o:(service::write_mutations_to_database(service::storage_proxy&, gms::inet_address, std::vector<canonical_mutation, std::allocator<canonical_mutation>>)) inarchive service/Dev/libservice.a
>>> referenced by group0_state_machine.cc
>>> group0_state_machine.cc.o:(service::write_mutations_to_database(service::storage_proxy&, gms::inet_address, std::vector<canonical_mutation, std::allocator<canonical_mutation>>) (.resume)) in archive service/Dev/libservice.a
>>> referenced 1 more times
```
Signed-off-by: Kefu Chai <kefu.chai@scylladb.com>
Closesscylladb/scylladb#18524
6047b3b6aa