xen: make dummy_shared_info non-static

Rename dummy_shared_info to xen_dummy_shared_info and make it
non-static, in anticipation of users outside of enlighten.c

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>