expose set_timestamp for runtime benchmarks (#8601)

* expose set_timestamp for runtime benchmarks

* Update frame/timestamp/src/lib.rs

Co-authored-by: Guillaume Thiolliere <gui.thiolliere@gmail.com>

Co-authored-by: Guillaume Thiolliere <gui.thiolliere@gmail.com>

substrate/frame/timestamp/src/lib.rs

@@ -258,7 +258,7 @@ impl<T: Config> Pallet<T> {
 	}
 
 	/// Set the timestamp to something in particular. Only used for tests.
-	#[cfg(feature = "std")]
+	#[cfg(any(feature = "runtime-benchmarks", feature = "std"))]
 	pub fn set_timestamp(now: T::Moment) {
 		Now::<T>::put(now);
 	}