Re-expose zbus' runtime features

Cargo.toml

@@ -10,6 +10,12 @@ repository = "https://github.com/hwchen/secret-service-rs.git"
 edition = "2018"
 version = "2.0.1"
 
+# The async runtime features mirror those of `zbus` for compatibility.
+[features]
+default = ["async-io"]
+async-io = ["zbus/async-io"]
+tokio = ["zbus/tokio"]
+
 [dependencies]
 # TODO: Update these when Rust 1.56 isn't so new.
 aes = "0.7.0"
@@ -20,4 +26,4 @@ num = "0.4.0"
 rand = "0.8.1"
 serde = { version = "1.0", features = ["derive"] }
 sha2 = "0.10.0"
-zbus = "2"
+zbus = { version = "2", default-features = false }