autoconf: xen: force a refresh with autoconf

autoconf: xen: force a refresh with autoconf

With some new functionality introduced we can't keep around the old cache,
but more specifically at times autogen.sh can produce inconsistent
results without considering all files obsolete. The cost of considering
all files obsolete is small so just force it.

Signed-off-by: Luis R. Rodriguez <mcgrof@suse.com>
Cc: Ian Campbell <ian.campbell@citrix.com>
Cc: Ian Jackson <ian.jackson@eu.citrix.com>
Cc: Jan Beulich <jbeulich@suse.com>
Cc: Keir Fraser <keir@xen.org>
Cc: Tim Deegan <tim@xen.org>
Acked-by: Roger Pau Monné <roger.pau@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>