[arcilator] Add missing install configuration for arcilator-runtime.h and arcilator-header-cpp.py

[FurryAcetylCoA]

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See #8212

The arcilator-runtime.h and python script are not installed and thus not included in pre-built releases.

I'm not entirely sure about the correct location for arcilator-runtime.h, but it seems that the header generated by arcilator-header-cpp.py expects arcilator-runtime.h to be located at the root of the include/ directory.

Additionally, the add_custom_target command is unnecessary and has been removed.

[FurryAcetylCoA]

friendly ping @maerhart