valgrind: use --time-stamp=yes

This doesn't help on the precise version, but maybe it will on
future versions... because if so, it would be really helpful
it matching valgrind errors to daemon logs!

In any case, though, I verified it is at worse harmless.

Signed-off-by: Sage Weil <sage@redhat.com>
Merge pull request #325 from ceph/wip-valgrind

valgrind: use --time-stamp=yes