ia64: fix compilation error

ia64: fix compilation error

This patch fixes the following compilation error.
Since struct page_list_head is defined in mm.h, sched.h needs mm.h.
Other circular inclusions are sorted out.

> In file included from xen/include/asm-ia64/linux-xen/asm/smp.h:50,
>                  from xen/include/linux/smp.h:5,
>                  from xen/include/asm-ia64/linux/topology.h:33,
>                  from xen/include/asm-ia64/linux-xen/linux/gfp.h:6,
>                  from xen/include/asm/mm.h:11,
>                  from xen/include/xen/mm.h:90,
>                  from viosapic.c:35:
> xen/include/xen/sched.h:174: error: field page_list has incomplete
> type
> xen/include/xen/sched.h:175: error: field xenpage_list has
> incomplete type

Signed-off-by: Isaku Yamahata <yamahata@valinux.co.jp>