KVM: define __KVM_HAVE_GUEST_DEBUG unconditionally

Since all architectures (for historical reasons) have to define
struct kvm_guest_debug_arch, and since userspace has to check
KVM_CHECK_EXTENSION(KVM_CAP_SET_GUEST_DEBUG) anyway, there is
no advantage in masking the capability #define itself.  Remove
the #define __KVM_HAVE_GUEST_DEBUG from architecture-specific
headers.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>