Declare read-only lookup pointers const

Declare read-only lookup pointers const

pwck.c:587:31: warning: cast discards ‘const’ qualifier from pointer target type [-Wcast-qual]
  587 |                         spw = (struct spwd *) spw_locate (pwd->pw_name);
      |                               ^

grpck.c:599:31: warning: cast discards ‘const’ qualifier from pointer target type [-Wcast-qual]
  599 |                         sgr = (struct sgrp *) sgr_locate (grp->gr_name);
      |                               ^

grpck.c:761:23: warning: cast discards ‘const’ qualifier from pointer target type [-Wcast-qual]
  761 |                 grp = (struct group *) gr_locate (sgr->sg_name);
      |                       ^