[Compile error]variable 'sigma_gn' set but not used [-Werror,-Wunused-but-set-variable]

```
leveldb/third_party/benchmark/src/complexity.cc:85:10: error: variable 'sigma_gn' set but not used [-Werror,-Wunused-but-set-variable]
double sigma_gn = 0.0;
^
1 error generated.

```

Compilation environment:

```
$ gcc --version
Apple clang version 13.1.6 (clang-1316.0.21.2)
Target: x86_64-apple-darwin21.3.0
Thread model: posix
InstalledDir: /Library/Developer/CommandLineTools/usr/bin
```