[hurd,commited] htl: Add missing fork.h

[Adhemerval Zanella]

On 24/03/2021 09:45, Samuel Thibault wrote:
> Adhemerval Zanella, le mer. 24 mars 2021 09:40:36 -0300, a ecrit:
>> Why exactly tst-atfork2 fails without this includes?
> 
> It misses the atfork unregister hook called at module unload.

Ok, but *why* exactly? The sysdeps/htl/pt-atfork.c already
includes register-atfork.h after the fork.h inclusion.  What
the missing includes misses to define on hurd implementation?