Fix broken opt folly build (-Werror,-Wunused-variable)

Summary: Fix broken opt folly build (-Werror,-Wunused-variable)

Reviewed By: yfeldblum

Differential Revision: D14593419

fbshipit-source-id: 5f298a2517fecfb33f8acf37b7d5ab9f714c934c

folly/experimental/pushmi/examples/reduce/reduce_2.cpp

@@ -136,6 +136,7 @@ int main() {
       2,
       std::plus<>{});
 
+  std::ignore = fortyTwo;
   assert(std::accumulate(vec.begin(), vec.end(), 2) == fortyTwo);
 
   std::cout << "OK" << std::endl;