xen: include xen/xen.h for definition of xen_initial_domain()

          CC      arch/x86/xen/setup.o
        arch/x86/xen/setup.c: In function 'xen_memory_setup':
        arch/x86/xen/setup.c:161: error: implicit declaration of function 'xen_initial_domain'

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>