Change: replace 'Unlicense/MIT' with 'Unlicense OR MIT' to make the license SPDX compliant

[jjnicola]

Issue #15

[wmmc88]

@BurntSushi Can you review this change? The license field being non-SPDX compliant(per https://doc.rust-lang.org/cargo/reference/manifest.html#the-license-and-license-file-fields) is also flagging in tools for me

[BurntSushi]

It seems like an odd thing to warn about given that Unlicense/MIT was at one point valid. I don't really get why it matters. But anyway, I've published winapi-util 0.1.8 to crates.io with this fixed.