Fix PostOp dependency

In general src headers should not be included in any public header of
other modules.

Since there are modules (graph, tests) that rely on specific PostOp definitions
in the previous src/core/experimental/PostOp.h, export it to the public
arm_compute header

Resolves COMPMID-4974

Signed-off-by: SiCongLi <sicong.li@arm.com>
Change-Id: I0fa4da5108a34fe6bfff1e9d57839da4e51dc314
Reviewed-on: https://review.mlplatform.org/c/ml/ComputeLibrary/+/6673
Reviewed-by: Gunes Bayir <gunes.bayir@arm.com>
Comments-Addressed: Arm Jenkins <bsgcomp@arm.com>
Tested-by: Arm Jenkins <bsgcomp@arm.com>
diff --git a/arm_compute/graph/backends/FunctionHelpers.h b/arm_compute/graph/backends/FunctionHelpers.h
index 55af056..1e420a8 100644
--- a/arm_compute/graph/backends/FunctionHelpers.h
+++ b/arm_compute/graph/backends/FunctionHelpers.h
@@ -24,6 +24,8 @@
 #ifndef ARM_COMPUTE_GRAPH_BACKENDS_DETAIL_FUNCTION_HELPERS_H
 #define ARM_COMPUTE_GRAPH_BACKENDS_DETAIL_FUNCTION_HELPERS_H
 
+#include "arm_compute/core/experimental/IPostOp.h"
+#include "arm_compute/core/experimental/PostOps.h"
 #include "arm_compute/graph/Logger.h"
 #include "arm_compute/graph/Tensor.h"
 #include "arm_compute/graph/TypePrinter.h"