Since e6fe2be3 (barvinok_summate: use isl interface,
Wed Nov 10 16:46:55 2010 +0100), only the functions declared
in <barvinok/isl.h> are used.  Now that this header is included
directly, the include of <barvinok/barvinok.h> can be removed.

Signed-off-by: Sven Verdoolaege <skimo@kotnet.org>