find the new RooFitJSONInterface component to fix compilation
The linker fails if the RooFitJSONInterface component is not included in find_package as a required component, because then the library is not added to the ROOT_LIBRARIES variable.
I'm not sure why it doesn't fail on other systems, but it fails for me currently (MacOS, using a ROOT master branch of 21 december). Searching for the component (which makes sense anyway) fixes compilation for me.