LogFilePrep: add a comment to the unsafe format string.

CVE-2018-14665 also made it possible to exploit this to access
memory. With -logfile forbidden when running with elevated privileges
this is no longer an issue.

Signed-off-by: Matthieu Herrb <matthieu@herrb.eu>
Reviewed-by: Adam Jackson <ajax@redhat.com>
(cherry picked from commit 248d164e)