Mark benchmark scripts as nightly-only

Author: bossmc

benches/body.rs

@@ -1,3 +1,4 @@
+#![cfg(feature = "nightly")]
 #![feature(test)]
 #![deny(warnings)]
 

benches/connect.rs

@@ -1,3 +1,4 @@
+#![cfg(feature = "nightly")]
 #![feature(test)]
 #![deny(warnings)]
 

benches/end_to_end.rs

@@ -1,3 +1,4 @@
+#![cfg(feature = "nightly")]
 #![feature(test)]
 #![deny(warnings)]
 

benches/pipeline.rs

@@ -1,3 +1,4 @@
+#![cfg(feature = "nightly")]
 #![feature(test)]
 #![deny(warnings)]
 

benches/server.rs

@@ -1,3 +1,4 @@
+#![cfg(feature = "nightly")]
 #![feature(test)]
 #![deny(warnings)]