[SYCL] Fix warnings: sub_group is struct not class

Signed-off-by: John Pennycook <[email protected]>

Author: Pennycook

sycl/include/CL/sycl/detail/type_traits.hpp

@@ -19,7 +19,7 @@ __SYCL_INLINE_NAMESPACE(cl) {
 namespace sycl {
 template <int Dimensions> class group;
 namespace intel {
-class sub_group;
+struct sub_group;
 } // namespace intel
 namespace detail {
 namespace half_impl {