rust: Suppress warning about improper C types

Zephyr takes advantage of a gcc/clang extension that allows structs that
have no elements.  Rust is perfectly happy with this (it is quite common
in Rust code), but generates a warning when a struct containing no
elements is passed to C code.

For now, suppress this warning on the generated bindings.  This has the
disadvantage of suppressing it entirely, which might possibly detect
other cases of invalid structs.  However, the bindings are
auto-generated from C structs so should always be valid.

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