clang-tidy - ignore third party headers code
The error you have posted does not look spurious. The problem likely isn't with the 3rd-party library, but with your usage of it. You have not supplied enough code to give a fully correct answer as it's not clear what fDummy and GetRandom are. If fDummy is being moved into GetRandom, you have a real error here.
The comments have already listed the ways to ignore errors contained within header files: using -isystem (in CMake, imported targets use this by default, otherwise you can manually apply SYSTEM to target_include_directories).