pgsql: Call srandom() instead of srand().

Log Message:
-----------
Call srandom() instead of srand().
pgbench calls random() later, so it should have called srandom().
On most platforms except Windows srandom() is actually identical
to srand(), so the bug only bites Windows users.
per bug report from Akio Ishida.

Modified Files:
--------------
    pgsql/contrib/pgbench:
        pgbench.c (r1.58 -> r1.59)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/contrib/pgbench/pgbench.c.diff?r1=1.58&r2=1.59)