Windows: missing ws2_32 when linking luasocket

Building for Windows with luasocket fails:

```
lld-link: error: undefined symbol: __declspec(dllimport) send
lld-link: error: undefined symbol: __declspec(dllimport) recv
...
```

luasocket's rockspec already declares the dependency:

```lua
modules["socket.core"].libraries = { "ws2_32" }
```

luast needs to read this `libraries` field and pass `-lws2_32` to the linker.