Resolve unexpected_cfgs warning

Cargo.toml

@@ -6,6 +6,7 @@ categories = ["asynchronous", "no-std"]
 description = "Type erasure for async trait methods"
 documentation = "https://docs.rs/async-trait"
 edition = "2021"
+exclude = ["build.rs"]
 keywords = ["async"]
 license = "MIT OR Apache-2.0"
 repository = "https://github.com/dtolnay/async-trait"

build.rs

@@ -0,0 +1,5 @@
+fn main() {
+    // Warning: build.rs is not published to crates.io.
+
+    println!("cargo:rustc-check-cfg=cfg(async_trait_nightly_testing)");
+}