kvm tools: arm: fixup pre-processor guards for kvm-cpu-arch.h

Cosmetic patch to correct the pre-processor guards for arm in
kvm-cpu-arch.h. It's not something to lose sleep over but I find it
incredibly irritating!

Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>