Two small haskell-related Makefile improvements

Just some cleanup:

- don't run hlint over hpc-htools, since it's just a symlink to
  htools.hs (so we'd get duplicate warnings)
- build rpc-test in make really-all, via HS_ALL_PROGS variable

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>