zpool wait: print timestamp before the header

list, status and iostat all display the -T timestamp before the header,
but wait showed it after. Make it be like the others.

Reported-by: Kyle Evans <kevans@FreeBSD.org>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Rob Norris <robn@despairlabs.com>
Closes #15825