perl: fix build of native modules

CORE/vutil.h is required when the host perl version is 5.20
see http://autobuild.buildroot.net/results/c3d/c3d48c3b3d6c1e616d8e20a2f65d8ba33abe61da/

Signed-off-by: Francois Perrad <francois.perrad@gadz.org>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>