Update time on Haskell binaries after build

This patch works around an issue in our build system. Since we don't
use cabal or 'ghc -M', we don't track actual dependencies in our
Makefile; this in turn means that editing a file that only is used in
the main 'htools' binary will keep triggering 'ghc --make' for the
hconfd binary; and this actually takes a bit of time to look at all
the dependencies.

So to work around this, we add a touch after each ghc --make, such
that we trigger the unneeded build at most once.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: René Nussbaumer <rn@google.com>