It used to cause errors if a thread model other than `posix` was selected,
which looks like a leftover from a7987858.

libgfortran/
	* io/async.h (struct async_unit): Use `__gthread_t` instead
	of `pthread_t`.

Signed-off-by: LIU Hao <lh_mouse@126.com>
Signed-off-by: Jonathan Yong <10walls@gmail.com>