Build failure on Windows

Hi, I noticed that the current version of this library no longer builds for Windows (i.e. with `GOOS=windows`):

```
/root/go/pkg/mod/github.com/zmap/zcrypto@v0.0.0-20250418211859-7510c141e4b7/x509/root.go:27:32: undefined: loadSystemRoots
```