Fix unused import `aesm-client` crate

[raoulstrackx]

Compilation of the aesm-client crate with the latest nightly fails with:

error: unused imports: `AesmError`, `Error`
  --> intel-sgx/aesm-client/src/imp/unix.rs:10:17
   |
10 | pub use error::{AesmError, Error, Result};
   |                 ^^^^^^^^^  ^^^^^
   |
note: the lint level is defined here
  --> intel-sgx/aesm-client/src/lib.rs:14:9
   |
14 | #![deny(warnings)]
   |         ^^^^^^^^
   = note: `#[deny(unused_imports)]` implied by `#[deny(warnings)]`

error: unused import: `AesmError`
 --> intel-sgx/aesm-client/src/imp/aesm_protobuf/mod.rs:2:17
  |
2 | pub use error::{AesmError, Error, Result};