Add a missing #ifdef HAVE_UNISTD_H
Add a missing #ifdef HAVE_UNISTD_H

Will be needed for native Windows build.

shekkbuilder/hwloc,ggouaillardet/hwloc,ggouaillardet/hwloc,shekkbuilder/hwloc,shekkbuilder/hwloc,ggouaillardet/hwloc,ggouaillardet/hwloc,shekkbuilder/hwloc