[libc++] Fix -Wgnu-include-next in stddef.h (#88214)

As reported in #86843, we must have #pragma GCC system_header before we
use #include_next, otherwise the compiler may not understand that we're
in a system header and may issue a diagnostic for our usage of

(cherry picked from commit 3c4b673)

Author: ldionne

libcxx/include/stddef.h

@@ -26,17 +26,17 @@
 
 #include <__config>
 
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#  pragma GCC system_header
+#endif
+
 // Note: This include is outside of header guards because we sometimes get included multiple times
 //       with different defines and the underlying <stddef.h> will know how to deal with that.
 #include_next <stddef.h>
 
 #ifndef _LIBCPP_STDDEF_H
 #  define _LIBCPP_STDDEF_H
 
-#  if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
-#    pragma GCC system_header
-#  endif
-
 #  ifdef __cplusplus
 typedef decltype(nullptr) nullptr_t;
 #  endif