x86/shadow: fix big-memory build

x86/shadow: fix big-memory build

Modifiers to the pointer passed into list_next_entry() are also being
applied to the macro's return type, and hence if the input pointer is
const-qualified a variable the result gets assigned to would also need
to be. As that doesn't seem desirable here, drop the const qualifier
on the input pointer instead.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Tim Deegan <tim@xen.org>