PR25118: move system_header pragma before uses of include_next to avoid extension warnings for people finding libc++ headers via -I paths.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@249788 91177308-0d34-0410-b5e6-96231b3b80d8

include/cstddef

@@ -34,14 +34,15 @@ Types:
 */
 
 #include <__config>
-// Don't include our own <stddef.h>; we don't want to declare ::nullptr_t.
-#include_next <stddef.h>
-#include <__nullptr>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #pragma GCC system_header
 #endif
 
+// Don't include our own <stddef.h>; we don't want to declare ::nullptr_t.
+#include_next <stddef.h>
+#include <__nullptr>
+
 _LIBCPP_BEGIN_NAMESPACE_STD
 
 using ::ptrdiff_t;