pam_krb5: 4.7 -> 4.8

Semi-automatic update generated by https://github.com/ryantm/nix-update tools. These checks were done:

- built on NixOS
- Warning: no binary found that responded to help or version flags. (This warning appears even if the package isn't expected to have binaries.)
- found 4.8 with grep in /nix/store/lbmm10zm3yif6n33xvw5irzhzsqkwk7b-pam-krb5-4.8
- directory tree listing: https://gist.github.com/8511cb10102508ce5701ed7e0830e60c