Fix a feature directive

Author: alexcrichton

src/lib.rs

@@ -65,7 +65,7 @@
 #![doc(html_root_url = "https://docs.rs/backtrace")]
 #![deny(missing_docs)]
 #![no_std]
-#![cfg_attr(target_env = "sgx", feature(sgx_platform))]
+#![cfg_attr(all(feature = "std", target_env = "sgx"), feature(sgx_platform))]
 #![allow(bare_trait_objects)] // TODO: remove when updating to 2018 edition
 #![allow(rust_2018_idioms)]   // TODO: remove when updating to 2018 edition