[Bug libstdc++/67408] assumes that __gthread_mutex_t and__gthread_recursive_mutex_t are the same types

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67408

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
I didn't think that code was usable on non-POSIX systems, and for POSIX they
are the same type. I'll see what I can do about it.