[SYCL][CUDA] Refactor __SYCL_INLINE macro

Synchronise the CUDA backend with the general SYCL changes from #1121.

Signed-off-by: Andrea Bocci <andrea.bocci@cern.ch>

Author: fwyzard

sycl/include/CL/sycl/backend/cuda.hpp

@@ -8,7 +8,7 @@
 
 #include <CL/sycl/detail/defines.hpp>
 
-__SYCL_INLINE namespace cl {
+__SYCL_INLINE_NAMESPACE(cl) {
 namespace sycl {
 namespace backend {
 namespace cuda {