KVM: Export include/linux/kvm.h only if $ARCH actually supports KVM

Currently, make headers_check barfs due to <asm/kvm.h>, which <linux/kvm.h>
includes, not existing.  Rather than add a zillion <asm/kvm.h>s, export kvm.h
only if the arch actually supports it.

Signed-off-by: Avi Kivity <avi@qumranet.com>