fix compilation with g++ 7.2.0 (#257)
fix compilation with g++ 7.2.0 (#257)

fails to compile on ubuntu 17.10
g++ (Ubuntu 7.2.0-8ubuntu3) 7.2.0
because of undefined std::function. adding the header fixes this.
istio/old_mixerclient_repo,istio/old_mixerclient_repo,istio/old_mixerclient_repo