The features in async-rt crate conflicts

[liqinggd]

The feature “sgx” and "auto_run" should be conflict features.

If Occlum links async-file with sgx feature, it will crash at thread '<unnamed>' panicked at 'The sgx thread policy must be Bound!', /root/testtt/deps/rust-sgx-sdk/sgx_tstd/src/thread/mod.rs:92:13