xen: Drop N from rcN in XEN_EXTRAVERSION

xen: Drop N from rcN in XEN_EXTRAVERSION

Having this here means we have to wait for a push gate pass, or fart
about which explicit pushes to master, to make an RC.  The boot
messages for git builds already contain the git revision (as a
shorthash).

I will change the tarball creation checklist to seddery the -rc back
to -rcN, along with the other release-management-related changes (like
using an embedded copy of qemu).

If this patch meets with approval it should be thrown into the push
gate today, along with the patch for XSA-88, and then hopefully
nothing much else, so that we can get something suitable for making an
RC from by Friday.

Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>
Acked-by: George Dunlap <george.dunlap@eu.citrix.com>
Release-Acked-by: George Dunlap <george.dunlap@eu.citrix.com>