Fix hiding of virtual function in FilterChainBenchmarkFixture

[nezdolik]

Currently Setup function in FilterChainBenchmarkFixture is hiding virtual function in parent class benchmark::Fixture :

   virtual void SetUp(State& st) { SetUp(const_cast<const State&>(st)); }
                ^~~~~
test/server/filter_chain_benchmark_test.cc:169:8: error:   by 'virtual void Envoy::Server::FilterChainBenchmarkFixture::SetUp(const benchmark::State&)' [-Werror=overloaded-virtual]
   void SetUp(const ::benchmark::State& state) override {
        ^~~~~

Risk Level: Low
Testing: Covered by existing tests