[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

GitOrigin-RevId: 3c4b673af05f53e8a4d1a382b5c86367ea512c9e
1 file changed