Add CROSS_PLATFORM_UUIDOF definition for all platforms

WinAdapter.h defines CROSS_PLATFORM_UUIDOF for non-WIN32 platforms
unless the compiler is clang.

include/dxc/dxcapi.h

@@ -23,7 +23,10 @@
 #endif
 #endif
 
-#ifdef _WIN32
+#ifndef _WIN32
+#include <dlfcn.h>
+#include "dxc/Support/WinAdapter.h"
+#endif
 
 #ifndef CROSS_PLATFORM_UUIDOF
 // Warning: This macro exists in WinAdapter.h as well
@@ -31,12 +34,6 @@
   struct __declspec(uuid(spec)) interface;
 #endif
 
-#else
-
-#include <dlfcn.h>
-#include "dxc/Support/WinAdapter.h"
-#endif
-
 struct IMalloc;
 
 struct IDxcIncludeHandler;