os: fix path/filepath test on Windows
os: fix path/filepath test on Windows

This is not the right fix, but it is what used to happen
before the FileInfo conversion, and it should get the
build working again (at least that part).

TBR=brainman
CC=golang-dev
http://codereview.appspot.com/5434090
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go