[boringssl] Update prerequisites for Rust bindgen

Change-Id: I936e272711524c05fc10cb73b19f106158a672f1
Reviewed-on: https://fuchsia-review.googlesource.com/c/third_party/boringssl/+/468334
Commit-Queue: Ali Zhang <alizhang@google.com>
Reviewed-by: Darren Krahn <dkrahn@google.com>
Fuchsia-Auto-Submit: Ali Zhang <alizhang@google.com>