Fix incorrect TRACE usages to use {str}

https://github.com/krb5/krb5/commit/3023a7d253c3052897b3d77d2f7ce992ff93e913
Author: Robbie Harwood <rharwood@redhat.com>
Committer: Greg Hudson <ghudson@mit.edu>
Commit: 3023a7d253c3052897b3d77d2f7ce992ff93e913
Branch: master
 src/plugins/preauth/pkinit/pkinit_trace.h |    6 +++---
 src/plugins/preauth/spake/trace.h         |    2 +-
 2 files changed, 4 insertions(+), 4 deletions(-)