gio: Don't redefine GKqueueFileMonitor

Fixes build on old compilers that don't allow type redefinitions.

Closes: #1506 (closed)

This builds for me on the old system that prompted me to file the above issue, and also still builds on the latest macOS 10.13. I can read C code but I don't often write it, so I had to Google how to fix this, and there might be something I don't understand about why it was done the way it was done before.