include: Add dummy field for Rust as well as CPP

For CPP builds, a few structs that end up empty in some configurations will
generate compile errors.  With the Rust tools, bindgen ends up producing
empty structsw for these as well.  Although the code compiles, it generates
warnings.  For now, add CONFIG_RUST to the ifdef checks so that these
structs don't end up empty with Rust either.

Signed-off-by: David Brown <david.brown@linaro.org>