[eldritchv2] Missing build.rs

[KCarretto]

When building for Windows (in our case SVC), you first need to compile the bin/reflective_loader crate manually, which is missing from our user docs and results in a confusing build error.

error: couldn't read `lib/eldritchv2/stdlib/eldritch-libsys/src/std/../../../../../../../bin/reflective_loader/target/x86_64-pc-windows-gnu/release/reflective_loader.dll`: No such file or directory (os error 2)
  --> lib/eldritchv2/stdlib/eldritch-libsys/src/std/dll_reflect_impl.rs:52:29
   |
52 |   const LOADER_BYTES: &[u8] = include_bytes!(concat!(
   |  _____________________________^
53 | |     "..",
54 | |     sep!(),
55 | |     "..",
...  |
77 | |     "reflective_loader.dll"
78 | | ));
   | |__^

https://docs.realm.pub/user-guide/imix#windows

[hulto]

I think this is solved by #1032