[IA64] stubs for populate on demand.

[IA64] stubs for populate on demand.

add stub functions of populate on demand to compile.
This patch fixes the following compile error.

> xen/common/built_in.o: In function `populate_physmap':
> xen/common/memory.c:116: undefined reference to `guest_physmap_mark_populate_on_demand'
> xen/common/built_in.o: In function `decrease_reservation':
> xen/common/memory.c:205: undefined reference to `p2m_pod_decrease_reservation'

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