COMPMID-959: Add missing header in Workload.h

Change-Id: I461a113246db73a5ff1e98f5f69cea6d84e2f91f
Reviewed-on: https://eu-gerrit-1.euhpc.arm.com/130689
Tested-by: Jenkins <bsgcomp@arm.com>
Reviewed-by: Pablo Tello <pablo.tello@arm.com>
diff --git a/arm_compute/graph/Workload.h b/arm_compute/graph/Workload.h
index 5ca9fd6..35066c4 100644
--- a/arm_compute/graph/Workload.h
+++ b/arm_compute/graph/Workload.h
@@ -26,6 +26,7 @@
 
 #include "arm_compute/runtime/IFunction.h"
 
+#include <functional>
 #include <memory>
 #include <vector>