make_lib_rust - use position independent code

[lynxcs]

added -fPIC when using make_lib_rust, since rust already does that by default.

This change is required by rust-kzg constantine backend to make c-kzg-4844 bindings work.

@mratsim if enabling for all rust builds is not acceptable, I can also add as optional feature in Cargo.toml & change how it's passed in build.rs

I am not familiar with Nim so if there's a better/more preferred way of doing this lmk, will be happy to fix.

[mratsim]

Thank you!

I would have done something similar.

[mratsim]

Seems like on Windows, -fPIC is not needed or even is an error with MinGW (what I use in CI):

• https://mingw-w64-public.narkive.com/4JUWn3PX/position-independent-code-fpic-on-x86-64-windows-dll-specially-for-cygwin
• Windows: -fPIC is added even platform is position independent jedisct1/libsodium#14
• make: Don't add fPIC if windows mia-0/rubberband#2