pgsql: Make the win32 implementation of getrusage() return EINVAL if

Log Message:
-----------
Make the win32 implementation of getrusage() return EINVAL if being
asked for anything other than RUSAGE_SELF, since it's not supported.

This is never called anywhere in the code today, but might be in
the future.

Not backpatching, since it's not called anywhere today.

Modified Files:
--------------
pgsql/src/port:
getrusage.c (r1.14 -> r1.15)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/port/getrusage.c?r1=1.14&r2=1.15)