Fix another portability bug in recent pgbench commit.

Fix another portability bug in recent pgbench commit.

Commit 547f04e7 produced errors on AIX/xlc while building plpython.  The
new code appears to be incompatible with the hack installed by commit
a11cf433.  Without access to an AIX system to check, my guess is that
_POSIX_C_SOURCE may be required for <time.h> to declare the things the
header needs to see, but plpython.h undefines it.

For now, to unbreak build farm animal hoverfly, just move the new
pg_time_usec_t support into pgbench.c.  Perhaps later we could figure
out what to rearrange to put it back into a header for wider use.

Discussion: https://postgr.es/m/CA%2BhUKG%2BP%2BjcD%3Dx9%2BagyTdWtjpOT64MYiGic%2Bcbu_TD8CV%3D6A3w%40mail.gmail.com