Do not accept empty definition of __XEN_INTERFACE_VERSION__

Do not accept empty definition of __XEN_INTERFACE_VERSION__
in xen-compat.h. It leads to building a broken kernel image
where the kernel sources end up using an unexpected interface
version. In the case of Linux, the kernel expects to use
the new sched_op() hypercall but ends up calling the
legacy hypercall -- this breaks poll, reboot, and save/restore.

A more acceptable patch would be to detect the empty
definition in xen-compat.h and give a reasonable #error message
to fail the build: the current error message is confusing.

Signed-off-by: Keir Fraser <keir@xensource.com>