LU-11830 libcfs: quiet print format warning

LU-11830 libcfs: quiet print format warning

The watchdog code was reworked in master but generates a warning on
b2_12 due to mismatched print format vs. time64_t.

Test-Parameters: trivial
Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Change-Id: I65beef9b5567c45802778afd78e3fde7299406f8
Reviewed-on: https://review.whamcloud.com/34805
Tested-by: Jenkins
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Ben Evans <bevans@cray.com>
Reviewed-by: Arshad Hussain <arshad.super@gmail.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>