[Runtime] NFC: remove C style "stddef.h" header from casting

Author: xedin

stdlib/public/runtime/Casting.cpp

@@ -21,7 +21,6 @@
 #include "ExistentialMetadataImpl.h"
 #include "Private.h"
 #include "SwiftHashableSupport.h"
-#include "stddef.h"
 #include "swift/Basic/LLVM.h"
 #include "swift/Basic/Lazy.h"
 #include "swift/Demangling/Demangler.h"