nailing-cargo: Provide a dummy build.rs

nailing-cargo: Provide a dummy build.rs

Without this, cargo might do this:

    package `<crate> <version> (/home/....)` specifies that it links
    to `<some library>` but does not have a custom build script

When generating a lockfile it doesn't actually need to look at the
build.rs so an empty one will do fine to placate it.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>