Add NFData instance for Tix

The hpc utility implements it's own Stricttypeclass here https://gitlab.haskell.org/ghc/ghc/-/blob/master/utils/hpc/HpcCombine.hs which is more or less isomorphic to NFData. This is probably due to the fact that hpc was implemented in ~2006 before NFData appeared. HPC-the-lib should implement the NFData instance, and HPC-the-util should use that instance.