[Bug libstdc++/88421] compat C headers break building GCC with GCC

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=88421

--- Comment #6 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to coypu from comment #1)
> suggested change: put #include_next outside of include guards?

We did something similar for PR 69581