C sources cannot include standard library headers

```
error(compilation): clang failed with stderr: third_party/zig/csource.c:1:10: fatal error: 'math.h' file not found

third_party/zig/csource.c:1:1: error: unable to build C object: clang exited with code 1
```

I'm on nixos 23.11, using bazel 6.4.0.