Add Threads to public CMake artefacts (#647)
Add Threads to public CMake artefacts (#647)

Threads is required by some targets which are exposed through the public
CMake configs. When they are attempted to be used, they will cause an
error if the including project doesn't call `find_package(Threads)`
themselves.


open-telemetry/opentelemetry-cpp,open-telemetry/opentelemetry-cpp,open-telemetry/opentelemetry-cpp,open-telemetry/opentelemetry-cpp