Fix rust build with stubs

[milesial]

In-between #477 and now, some refactor dropped the actual binding generation when using stubs, leading to an error

$ cargo build --features stub-api
...
warning: nixl-sys@0.7.1: Building with stub API - NIXL functions will abort if called
error: couldn't read `/data/nixl/target/debug/build/nixl-sys-091d69c4bbf874e2/out/bindings.rs`: No such file or directory (os error 2)
  --> src/bindings/rust/src/lib.rs:39:5
   |
39 |     include!(concat!(env!("OUT_DIR"), "/bindings.rs"));
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

warning: nixl-sys@0.7.1: Building with stub API - NIXL functions will abort if called
error: could not compile `nixl-sys` (lib) due to 1 previous error

This PR fixes that.

[jthomson04]

Not for this PR, but we definitely need some CI test for the stub api

[roiedanino]

I think it can be a great idea to add

cargo build --features stub-api

in .gitlab/test_rust.sh

I think it can be added in this PR, it's just a single line of change

[gleon99]

I think it can be a great idea to add

cargo build --features stub-api

in .gitlab/test_rust.sh

I think it can be added in this PR, it's just a single line of change

+1

[milesial]

I think it can be a great idea to add

cargo build --features stub-api

in .gitlab/test_rust.sh

I think it can be added in this PR, it's just a single line of change

Added

[milesial]

@roiedanino can this be merged?