Fix cross compilation to windows from linux

`Winsock2.h` should be all lowercase, then compilation works.

```
amd64.windows.gcc.exe       = "x86_64-w64-mingw32-gcc"
amd64.windows.gcc.linkerexe = "x86_64-w64-mingw32-gcc"

i386.windows.gcc.exe        = "i686-w64-mingw32-gcc"
i386.windows.gcc.linkerexe  = "i686-w64-mingw32-gcc"
```

also needs to be added to `config/nim.cfg`, but I'm not sure if that's consistent between systems.