rust: add bindgen step as a meson dependency

rust: add bindgen step as a meson dependency

Add bindings_rs target for generating rust bindings to target-independent
qemu C APIs.

The bindings need be created before any rust crate that uses them is
compiled.

The bindings.rs file will end up in BUILDDIR/bindings.rs and have the
same name as a target:

  ninja bindings.rs

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Manos Pitsidianakis <manos.pitsidianakis@linaro.org>
Link: https://lore.kernel.org/r/1be89a27719049b7203eaf2eca8bbb75b33f18d4.1727961605.git.manos.pitsidianakis@linaro.org
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>