Get build script to compile on windows again.

It looks like this was just a simple typo, or maybe a failed merge.

I think there is more work to be done to get this crate to actually compile on windows, but this is a first step.

Some questions/comments:

• I'm pretty sure link_paths and all the other properties in nettle: Library are being wrongly ignored?
• https://github.com/ShiftMediaProject/nettle and https://github.com/gx/gmp seem to at least partially work to get this crate to compile, is that how it was meant to work on Windows? It seems a bit weird since I'm handwriting the vcpkg nettle.pc and hogweed.pc, are there better versions I can use?