[oneDPL] Declare predefined dpcpp_default policy as const (#554)

Author: SergeyKopienko

source/elements/oneDPL/source/parallel_api.rst

@@ -65,7 +65,7 @@ where and how an algorithm runs.
         template <typename KernelName = /*unspecified*/>
         class device_policy;
 
-        device_policy<> dpcpp_default;
+        const device_policy<> dpcpp_default;
 
         template <typename KernelName = /*unspecified*/>
         device_policy<KernelName>