lib/Makefile.am: Uninstall ivykis headers when needed

When we installed the ivykis headers (if using the internal ivykis), we
should uninstall them too. This hooks up an uninstall hook that does
just that.

Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>
Merge pull request #136 from algernon/f/3.5/internal-ivykis-install

lib/Makefile.am: Uninstall ivykis headers when needed