Make dependency optional

Author: danog

Cargo.toml

@@ -18,7 +18,7 @@ cfg-if = "1.0"
 once_cell = "1.17"
 anyhow = { version = "1", optional = true }
 ext-php-rs-derive = { version = "=0.10.1", path = "./crates/macros" }
-php-tokio = "0.1.4"
+php-tokio = { version = "=0.1.4", optional = true }
 
 [dev-dependencies]
 skeptic = "0.13"
@@ -41,7 +41,7 @@ zip = "0.6"
 [features]
 closure = []
 embed = []
-async = []
+async = ["dep:php-tokio"]
 
 [workspace]
 members = ["crates/macros", "crates/cli"]