package/xenomai: disable obstack support

package/xenomai: disable obstack support

Obstack support in Xenomai is broken when the libc doesn't support
obstack support itself. Make sure we disable obstack support for all
libc since it was removed in uClibc-ng since 1.0.21 release and it's
always disabled with Glibc.

Fixes:
http://autobuild.buildroot.net/results/555/555d9cb9cb59fad6353516bed206e1117974e8cd/build-end.log

Signed-off-by: Romain Naour <romain.naour@gmail.com>
Cc: Pawel Sikora <sikor6@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>