Prusa i3 Rework - X Motor Mount with Magnet Holder for Hall Endstop

Added a small holder for a magnet for a hall endstop. I'm using the endstop bracket here: https://www.thingiverse.com/thing:145941

The SCAD files are at my fork at https://github.com/garyjohnson/Prusa3

Just superglue the magnet in and you're good to go!