buildroot

Revert "package/xenomai: disable obstack support"

This reverts commit e87644cb2abb78c5bf3bd0c3c6955c202e798ef2.

The issue is now fixed in Xenomai, let use Xenomai obstack support
if it's not already available from the libc.

Signed-off-by: Romain Naour <romain.naour@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>