[SYCL][PI] Treat SYCL headers as system headers

SYCL examples, e.g., in the Khronos specs, include the SYCL header as
system headers.

Adapt includes in the `CL/sycl/detail/pi.hpp` header to this style.

Signed-off-by: Bjoern Knafla <[email protected]>

Author: bjoernknafla

sycl/unittests/pi/PlatformTest.cpp

@@ -6,8 +6,8 @@
 //
 //===----------------------------------------------------------------------===//
 
-#include "CL/sycl/detail/pi.hpp"
 #include <CL/sycl.hpp>
+#include <CL/sycl/detail/pi.hpp>
 #include <gtest/gtest.h>
 #include <memory>