pgsql: Set stderr to unbuffered for pgbench on win32.

Log Message:
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Set stderr to unbuffered for pgbench on win32.
ITAGAKI Takahiro

Modified Files:
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pgsql/contrib/pgbench:
pgbench.c (r1.61 -> r1.62)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/contrib/pgbench/pgbench.c.diff?r1=1.61&r2=1.62)