fix rust-random#688

Author: newpavlov

Cargo.toml

@@ -36,4 +36,4 @@ fuchsia-zircon = "0.3.2"
 
 [target.'cfg(target_env = "sgx")'.dependencies]
 rdrand = "0.4.0"
-rand_core = "0.3.0"
+rand_core = { version = "0.3", default-features = false }