[benchmark] Silence 'suggest override' and 'missing override' warnings

When building with Clang 11 on Windows, silence the following:

F:\aganea\llvm-project\llvm\utils\benchmark\include\benchmark/benchmark.h(955,8): warning: 'Run' overrides a member function but is not marked 'override' [-Wsuggest-override]
  void Run(State& st);
       ^
F:\aganea\llvm-project\llvm\utils\benchmark\include\benchmark/benchmark.h(895,16): note: overridden virtual function is here
  virtual void Run(State& state) = 0;
               ^
1 warning generated.