Fix compiler error in CLActivationLayer

* GCC 7.4 + Ubuntu 16
* Resolves COMPMID-4761.

Change-Id: Id259f00c860b23c4a20881dd0c6ab6468cf17385
Signed-off-by: Pablo Marquez Tello <pablo.tello@arm.com>
Reviewed-on: https://review.mlplatform.org/c/ml/ComputeLibrary/+/6070
Tested-by: Arm Jenkins <bsgcomp@arm.com>
Reviewed-by: Nikhil Raj Arm <nikhil.raj@arm.com>
diff --git a/src/runtime/gpu/cl/operators/ClActivation.cpp b/src/runtime/gpu/cl/operators/ClActivation.cpp
index bef42d7..34a2f94 100644
--- a/src/runtime/gpu/cl/operators/ClActivation.cpp
+++ b/src/runtime/gpu/cl/operators/ClActivation.cpp
@@ -69,7 +69,7 @@
     if(op == nullptr)
     {
         ARM_COMPUTE_LOG_ERROR_ACL("Couldn't allocate internal resources");
-        return { nullptr, StatusCode::OutOfMemory };
+        return std::make_tuple(nullptr, StatusCode::OutOfMemory);
     }
     op->set_internal_operator(std::move(act_op));