build-system/haskell: install config for any package that creates it.

A Cabal package is allowed to declare an "empty" library, in an
otherwise executable-only package, for the purpose of allowing Cabal
to use it as a dependency for other packages.  See e.g. hspec-discover.

* guix/build/haskell-build-system.scm (register): Unconditionally call
  setup script with "register", and install any config file generated.