Call tzset() before localtime_r() as the docs say.

Without it, one might run the risk of QDateTime::currentDateTime()
returning an invalid QDateTime the first time after changing timezone.

Change-Id: I3efb04d41e7fe4685f6cc5fb41b68424eb4b9eb8
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>