xenomai: bump to 3.0.6

xenomai: bump to 3.0.6

This new version includes a fix to allow compilation with compilers
defaulting to -fpie (gcc 6+). It also a fix for a critical bug in
Cobalt:

http://xenomai.org/pipermail/xenomai/2017-November/037923.html

Signed-off-by: Norbert Lange <nolange79@gmail.com>
Reviewed-by: Romain Naour <romain.naour@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
(cherry picked from commit 34d20f0b878c4f18ee5f5af0276894499d3f9bf8)
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>