IVGCVSW-2865 add missing include of functional

Change-Id: Ib110f8fe8b8980ad948d2ca22061e9d020cb33d7
Signed-off-by: Matthew Bentham <matthew.bentham@arm.com>
diff --git a/include/armnn/Types.hpp b/include/armnn/Types.hpp
index 693a050..bdddd37 100644
--- a/include/armnn/Types.hpp
+++ b/include/armnn/Types.hpp
@@ -5,6 +5,7 @@
 #pragma once
 
 #include <array>
+#include <functional>
 #include <memory>
 #include "BackendId.hpp"
 #include "Exceptions.hpp"