[IA64] compilation fix of c/s 17723:c684cf331f94

[IA64] compilation fix of c/s 17723:c684cf331f94

This patch fixes the compilation error caused by the c/s 17723:c684cf331f94.
Define stub functions which return -ENOSYS.

ia64-linux-gnu-ld -g -T xen.lds.s -N \
                -Map map.out linux-xen/head.o xen-unstable.hg/xen/common/built_in.o xen-unstable.hg/xen/drivers/built_in.o xen-unstable.hg/xen/xsm/built_in.o xen-unstable.hg/xen/arch/ia64/built_in.o \
                xen-unstable.hg/xen/common/symbols-dummy.o -o xen-unstable.hg/xen/xen-syms
xen-unstable.hg/xen/common/built_in.o: In function `__gnttab_unmap_common':
xen-unstable.hg/xen/common/grant_table.c:566: undefined reference to `iommu_unmap_page'
xen-unstable.hg/xen/common/built_in.o: In function `__gnttab_map_grant_ref':
xen-unstable.hg/xen/common/grant_table.c:388: undefined reference to `iommu_map_page'

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