compile with target bpfel-unknown-none

[Sherlock-Holo]

when set target to bpfel-unknown-none, cargo build will failed

error[E0432]: unresolved import `ad`
  --> /home/sherlock/.cargo/registry/src/github.com-1ecc6299db9ec823/cty-0.2.2/src/lib.rs:13:9
   |
13 | pub use ad::*;
   |         ^^ maybe a missing crate `ad`?
   |
   = help: consider adding `extern crate ad` to use the `ad` crate

For more information about this error, try `rustc --explain E0432`.
error: could not compile `cty` due to previous error
warning: build failed, waiting for other jobs to finish...