Remove unknown flag for gcc12
Remove unknown flag for gcc12

It fails as:
```
cc1plus: note: unrecognized command-line option '-Wno-unknown-warning-option' may have been intended to silence earlier diagnostics
```

thepowersgang/mrustc,thepowersgang/mrustc,thepowersgang/mrustc,thepowersgang/mrustc,thepowersgang/mrustc