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

NervanaSystems/ngraph,NervanaSystems/ngraph,NervanaSystems/ngraph,NervanaSystems/ngraph