xen: annotate xen_set_identity_and_remap_chunk() with __init

Commit 5b8e7d80

 removed the __init
annotation from xen_set_identity_and_remap_chunk(). Add it again.

Signed-off-by: Juergen Gross <jgross@suse.com>
Signed-off-by: David Vrabel <david.vrabel@citrix.com>