self-signed end entity cert is not trusted when added as extra root.

https://github.com/rustls/rustls-platform-verifier/blob/93f729c6309680486d33f982e3ba3c42b976f774/rustls-platform-verifier/src/verification/windows.rs#L659

Under windows, extra roots are added only when TrustStatus.dwErrorStatus is CERT_TRUST_IS_PARTIAL_CHAIN. But the error is CERT_TRUST_IS_UNTRUSTED_ROOT when the cert is self-signed and the issuer is itself. 