Fix compilation warnings when building against hurd-amd64.

Passes mach_msg_type_number_t instead of size_t which have different
sizes. Example warning:

/hurd_pci.c:101:53: warning: passing argument 3 of 'pci_get_dev_regions' from incompatible pointer type [-Wincompatible-pointer-types]
  101 |     err = pci_get_dev_regions(d->device_port, &buf, &size);
      |                                                     ^~~~~
      |                                                     |
      |                                                     size_t * {aka long unsigned int *}