Add missing && when forwarding args (#925)

Without the && args would be copied and perfect forwarding won't work.

benchmarks/cpp/time_utils.h

@@ -24,7 +24,7 @@
             << std::endl;
 
 template <typename F, typename... Args>
-double time_fn(F fn, Args... args) {
+double time_fn(F fn, Args&&... args) {
   // warmup
   for (int i = 0; i < 5; ++i) {
     eval(fn(std::forward<Args>(args)...));