benchmark

src/benchmark_register.h: add missing <limits> inclusion (#1060)

Noticed missing header when was building llvm with gcc-11:

```
llvm-project/llvm/utils/benchmark/src/benchmark_register.h:17:30:
  error: 'numeric_limits' is not a member of 'std'
   17 |   static const T kmax = std::numeric_limits<T>::max();
      |                              ^~~~~~~~~~~~~~
```