[SYCL][CUDA] Inline cl namespace to simplify SYCL API usage

Synchronise the CUDA backend with the general SYCL changes from #974.

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

Author: fwyzard

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

@@ -6,7 +6,9 @@
 //
 //===----------------------------------------------------------------------===//
 
-namespace cl {
+#include <CL/sycl/detail/defines.hpp>
+
+__SYCL_INLINE namespace cl {
 namespace sycl {
 namespace backend {
 namespace cuda {