xvisor: fix bogus check on XVISOR_ARCH

xvisor: fix bogus check on XVISOR_ARCH

The XVISOR_ARCH check added in commit
117fd5dfbc756c4f2b4aef97fc2b568528c66df7 ("xvisor: fix build on
AArch64") broke Buildroot entirely on all architectures except ARM,
AArch64 and x86-64, because the $(error ...) test was not enclosed
inside a condition that made sure the xvisor package was enabled.

This commit fixes that, and allows Buildroot to be usable again on all
architectures.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>