package/xenomai: fix latest version
The last xenomai version bump [1] forgot to update the
BR2_PACKAGE_XENOMAI_LATEST_VERSION option string.
[1] 44fafcc41aca4c32ecd937c4b7ae93b7a713d9f2
Reported-by: Pierre Ficheux <pierre.ficheux@smile.fr>
Signed-off-by: Romain Naour <romain.naour@smile.fr>