pam_ldap: fix build

an upstream perl script needed during build wasn't found, fix path

pkgs/os-specific/linux/pam_ldap/default.nix

···
 
   postPatch = ''
     patchShebangs ./vers_string
+    substituteInPlace vers_string --replace "cvslib.pl" "./cvslib.pl"
   '';
 
   preInstall = "