fixed classpathresolver again for windows

Hi,
Today, I have tried gradlew runModEclipse, but failed it.
The previous [COMMIT](https://github.com/vert-x/vert.x/commit/9f7e37b2472d5ed98a2396dbd23d340c227a3407)　was not enough.
Since I wasn't on top of making sure of things, I overlooked that, sorry.