LibC: Tolerate stddef.h being included from the kernel

This can get pulled in by <cxxabi.h>, but we don't want it pulling in
more stuff from LibC.

Libraries/LibC/stddef.h

@@ -1,5 +1,7 @@
 #pragma once
 
+#ifndef KERNEL
+
 #include <sys/cdefs.h>
 #include <sys/types.h>
 
@@ -11,3 +13,5 @@
 
 typedef __PTRDIFF_TYPE__ ptrdiff_t;
 typedef __SIZE_TYPE__ size_t;
+
+#endif