rpc_tester: job_cpu: mark virtual methods override
rpc_tester: job_cpu: mark virtual methods override

Fix build error with clang 12.0.1 c++17:
```
../../apps/rpc_tester/rpc_tester.cc:367:25: error: 'name' overrides a member function but is not marked 'override' [-Werror,-Winconsistent-missing-override]
    virtual std::string name() const { return _cfg.name; }
                        ^
```

Signed-off-by: Benny Halevy <23c997c90eb8537635e9392d48e7af1bf1b2ca22@scylladb.com>
Message-Id: <20220417143654.1869975-1-23c997c90eb8537635e9392d48e7af1bf1b2ca22@scylladb.com>

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