Fix applyPreset signal so assign-colors works
The SIGNAL macro turns everything into a string, so the name needs to match the name-mangled one vtk uses, vtkJson.
I'm puzzled why this is needed now, and not with PV 5.6, because the json.h header has the same name-mangling #define in it.
@bob.obara LMK if you think this is correct.
Fixes #293 (closed)