pgbench --progress-timestamp no longer works correctly

--progress-timestamp is supposed to make -P report a Unix Epoch time stamp,
for easy correlation with the entries in other log files (like the postgres
server log file using %n).

But that broke in this commit:

commit 1d63f7d2d180c8708bc12710254eb7b45823440f
Author: Tom Lane <tgl@sss.pgh.pa.us>
Date:   Mon Jan 2 13:41:51 2017 -0500

    Use clock_gettime(), if available, in instr_time measurements.


The commit before that one changed pgbench to make it tolerate the change
in clock, but it overlooked --progress-timestamp.

Cheers,

Jeff