Fix sgx_tstd xargo compile, due to upstream change
diff --git a/sgx_tstd/src/lib.rs b/sgx_tstd/src/lib.rs
index 74fe1ea..8385831 100644
--- a/sgx_tstd/src/lib.rs
+++ b/sgx_tstd/src/lib.rs
@@ -68,7 +68,6 @@
 #![feature(c_variadic)]
 #![feature(cfg_target_has_atomic)]
 #![feature(char_error_internals)]
-#![feature(compiler_builtins_lib)]
 #![feature(concat_idents)]
 #![feature(const_fn)]
 #![feature(core_intrinsics)]