pgsql: 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

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/c427de427ac411039d5efd5d0dcc8a4e0c6da68d

Modified Files
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src/bin/pgbench/pgbench.c | 25 +++++++++++++++++++++++++
src/include/portability/instr_time.h | 28 ----------------------------
2 files changed, 25 insertions(+), 28 deletions(-)