Allow header-only libraries in FindxDAQ.cmake

Summary

Currently, header-only libraries are not supported in FindxDAQ.cmake. This creates a delicate situation such as this one with nlohmann. If the library was to not be installed, the compilation would fail with an ugly error message. That should, and must, be caught during the configure step.

Moreover, contrarily to all other packages, it is not clear that the corresponding package must be installed in the CI.

What is the expected correct behavior?

Header-only library, such as nlohmann, should be supported by FindxDAQ.cmake.