Fixes compile errors on gcc 7.3. (#628)

src/ngraph/graph_util.hpp

@@ -16,6 +16,7 @@
 
 #pragma once
 
+#include <functional>
 #include <list>
 #include <memory>
 #include <string>

src/ngraph/pass/assign_placement.hpp

@@ -17,6 +17,7 @@
 #pragma once
 
 #include <exception>
+#include <functional>
 #include <sstream>
 
 #include "ngraph/pass/pass.hpp"

src/ngraph/runtime/cpu/pass/cpu_nop_elimination.cpp

@@ -14,6 +14,7 @@
 * limitations under the License.
 *******************************************************************************/
 
+#include <functional>
 #include <memory>
 #include <typeindex>
 #include <typeinfo>

src/ngraph/runtime/manager.cpp

@@ -15,6 +15,7 @@
 *******************************************************************************/
 
 #include <dlfcn.h>
+#include <functional>
 #include <iostream>
 #include <sstream>
 #include <string>

src/ngraph/runtime/manager.hpp

@@ -16,6 +16,7 @@
 
 #pragma once
 
+#include <functional>
 #include <map>
 #include <memory>
 #include <string>