Make ksu work with prompting clpreauth modules

Commit 5fd5a67c5a93514e7d0a64425baa007ad91f57de switched ksu from
using krb5_get_in_tkt_with_password() to
krb5_get_init_creds_password(), but did not supply a prompter
argument.  Pass krb5_prompter_posix so that clpreauth modules can
prompt for additional information during authentication.

https://github.com/krb5/krb5/commit/23a16fb5eac733880e34a770882ed17b93b5d66c
Author: Greg Hudson <ghudson@mit.edu>
Commit: 23a16fb5eac733880e34a770882ed17b93b5d66c
Branch: master
 src/clients/ksu/krb_auth_su.c |    3 ++-
 1 files changed, 2 insertions(+), 1 deletions(-)