COMPMID-1478: Stop relying on static default OpenCL objects in cl2.hpp

This causes problems when ACL is used as a shared library on Android.
Fixes some problems related to creation / destruction order between the Graph's CL backend and core / runtime

Change-Id: I716d63fd42f4586df1ffbb6fa97e4db06d3a781b
Reviewed-on: https://eu-gerrit-1.euhpc.arm.com/143228
Tested-by: Jenkins <bsgcomp@arm.com>
Reviewed-by: Michele DiGiorgio <michele.digiorgio@arm.com>
Reviewed-by: Gian Marco Iodice <gianmarco.iodice@arm.com>
diff --git a/arm_compute/core/CL/OpenCL.h b/arm_compute/core/CL/OpenCL.h
index 3305330..468e179 100644
--- a/arm_compute/core/CL/OpenCL.h
+++ b/arm_compute/core/CL/OpenCL.h
@@ -82,6 +82,7 @@
 #define DECLARE_FUNCTION_PTR(func_name) \
     std::function<decltype(func_name)> func_name##_ptr = nullptr
 
+    DECLARE_FUNCTION_PTR(clCreateContext);
     DECLARE_FUNCTION_PTR(clCreateContextFromType);
     DECLARE_FUNCTION_PTR(clCreateCommandQueue);
     DECLARE_FUNCTION_PTR(clGetContextInfo);