Problem: Compile error on Mac with gcc 7.2 (#270)

* Problem: Compile error on Mac with gcc 7.2
Solution: Should include <functional> to use std::function.

* Sort include names in alphabetical order.

src/ngraph/util.hpp

@@ -16,6 +16,7 @@
 
 #include <algorithm>
 #include <chrono>
+#include <functional>
 #include <iostream>
 #include <map>
 #include <memory>