[SYCL] Prefer _WIN32 over WIN32 in #if expressions (#16267)

The former is defined by the toolchain while the latter is part of SDK,
so using `_WIN32` is preferrable. The issue was exposed downstream due
to recent changes from #16178 and
#16194.

Author: aelovikov-intel

sycl/include/sycl/ext/intel/experimental/pipes.hpp

@@ -69,7 +69,7 @@ class pipe_base {
 #ifdef __SYCL_PIPES_CPP
 // Magic combination found by trial and error:
 __SYCL_EXPORT
-#ifdef WIN32
+#if _WIN32
 inline
 #endif
 #else

sycl/include/sycl/ext/oneapi/experimental/graph.hpp

@@ -388,7 +388,7 @@ class __SYCL_EXPORT modifiable_command_graph {
 #ifdef __SYCL_GRAPH_IMPL_CPP
 // Magic combination found by trial and error:
 __SYCL_EXPORT
-#if WIN32
+#if _WIN32
 inline
 #endif
 #else