Install missing files on Release 2.6 branch

In DORiE we are trying to set up Spack package for easy deplyoment. For this, one has to install all dependent modules with Spack as well. Packaging dune-randomfield is really easy! (See dorie-spack/packages/dune-randomfield). Doing so, I noticed that dune-randomfield does not install all the header files and the CMake modules. Without this, downstream modules cannot be installed without the dune-randomfield build directory. This MR should fix, at least, the obvious things for releases/2.6 branch.