[futility] Fix build target.

This fixes the `host_prebuilts-x64-linux` builder, which has been
failing for over 2 months due to a clang roll[1], which picked up [2],
introducing `-Wuninitialized-const-pointer`.

[1]: https://fxrev.dev/1327414
[2]: https://github.com/llvm/llvm-project/commit/00dacf8c22f065cb52efb14cd091d441f19b319e

Fixed: 441752797
Change-Id: Id6564847cee992ae1afe29a39f64698d42c55773
Reviewed-on: https://fuchsia-review.googlesource.com/c/third_party/vboot_reference/+/1357324
Reviewed-by: Jiaming Li <lijiaming@google.com>
Reviewed-by: Ina Huh <ihuh@google.com>