Your best bet is either a standoff. Try a trip to the hardware store to find a long (2-3 cm) bolt that has one end with a female thread and one end with a male thread. The male thread gets threaded into the eyelet and the female thread accepts the mounting bolt of the fender stay. The 2-3 cm should get the stay out far enough to clear the disc brake caliper.

The ghetto solution would be to use a longer bolt and a stack of washers.

You may also be able to rig up something if your bike has rack eyelets.