package/xenomai: ignore custom versions

We can safely reference XENOMAI_SOURCE, even though it is defined later,
because BR_NO_CHECK_HASH_FOR is a recursively-expanded variable, so its
content is only evaluated when the variable is referenced.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Peter Korsgaard <jacmet@uclibc.org>
Acked-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>