deps: enable only the std feature for the futures crate

aya/Cargo.toml

@@ -12,7 +12,7 @@ bitflags = "1.2.1"
 bytes = "1"
 lazy_static = "1"
 parking_lot = { version = "0.11.1", features = ["send_guard"] }
-futures = { version = "0.3.12", optional = true }
+futures = { version = "0.3.12", optional = true, default-features = false, features = ["std"] }
 tokio = { version = "1.2.0", features = ["macros", "rt", "rt-multi-thread", "net"], optional = true }
 async-std = { version = "1.9.0", optional = true }
 async-io = { version = "1.3", optional = true }