addons headers are not properly installed

[tmadlener]

I ran into this because EDM4hep seems to try and pick up the shared libraries/dictionaries that are built by FCCAnalyses in CI:
https://github.com/key4hep/EDM4hep/actions/runs/3468331195/jobs/5794042094#step:4:589

The problem seems to be that the FastJet/JetClustering.h is not installed. I didn't yet check very thoroughly, but it seems that none of the header files in the addons/FastJet folder are installed to prefix/include.

[tmadlener]

Also the CMake config needs adaptation most likely:

FCCAnalyses/analyzers/dataframe/CMakeLists.txt

Line 42 in 8ce567f

$<BUILD_INTERFACE:${CMAKE_SOURCE_DIR}/addons>

Alternatively if these are private headers, they should be declared as such (and removed from the public headers)