xenomai: fix build failure when HAVE_OPEN64 is undefined

fixes: http://autobuild.buildroot.org/results/d59/d590be34637a4043c929ce615f9db504484be435/build-end.log

Upstream patch:
http://git.xenomai.org/xenomai-2.6.git/commit/?id=71345fe15f126ae3a596af309945d0af6edfbbbb



Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Romain Naour <romain.naour@openwide.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>