fix(examples): "cargo test --features serde-serialization"

Author: erickt

examples/headers.rs

@@ -4,6 +4,9 @@
 // TODO: only import header!, blocked by https://github.com/rust-lang/rust/issues/25003
 extern crate hyper;
 
+#[cfg(feature = "serde-serialization")]
+extern crate serde;
+
 // A header in the form of `X-Foo: some random string`
 header! {
     (Foo, "X-Foo") => [String]