Correct formatting of trace log microseconds

Always use six digits with leading 0s to format the microseconds in
trace log timestamps; otherwise a small value appears as too large of
a fraction of a second.

https://github.com/krb5/krb5/commit/734bf341da54e09add9160e65ea7308072b97f13
Author: Greg Hudson <ghudson@mit.edu>
Commit: 734bf341da54e09add9160e65ea7308072b97f13
Branch: master
 src/lib/krb5/os/trace.c |    4 ++--
 1 files changed, 2 insertions(+), 2 deletions(-)