After the previous fix in 6f1f34c9 msvc ended up looking for psqlscan.c
in the wrong directory.

David's fix just forces the path to be adjusted. That's not a
particularly pretty fix, but it hopefully will make the buildfarm green
again.

Author: David Rowley
Discussion: CAKJS1f_9CCi_t+LEgV5GWoCj3wjavcMoDc5qfcf_A0UwpQoPoA@mail.gmail.com