ia64: fix build failure caused by memory model changes

The change of ia64's default memory model to SPARSEMEM causes defconfig
build to fail:

  CC      kernel/async.o
In file included from include/linux/numa.h:25,
                 from include/linux/async.h:13,
                 from kernel/async.c:47:
arch/ia64/include/asm/sparsemem.h:14:40: warning: "PAGE_SHIFT" is not defined, evaluates to 0 [-Wundef]
   14 | #if ((CONFIG_FORCE_MAX_ZONEORDER - 1 + PAGE_SHIFT) > SECTION_SIZE_BITS)
      |                                        ^~~~~~~~~~
In file included from include/linux/gfp.h:6,
                 from include/linux/xarray.h:14,
                 from include/linux/radix-tree.h:19,
                 from include/linux/idr.h:15,
                 from include/linux/kernfs.h:13,
                 from include/linux/sysfs.h:16,
                 from include/linux/kobject.h:20,
                 from include/linux/energy_model.h:7,
                 from include/linux/device.h:16,
                 from include/linux/async.h:14,
                 from kernel/async.c:47:
include/linux/mmzone.h:1156:2: error: #error Allocator MAX_ORDER exceeds SECTION_SIZE
 1156 | #error Allocator MAX_ORDER exceeds SECTION_SIZE
      |  ^~~~~

The error cause is the missing definition of PAGE_SHIFT in the calculation
of SECTION_SIZE_BITS.

Add include of <asm/page.h> to arch/ia64/include/asm/sparsemem.h to solve
the problem.

Fixes: 214496cb ("ia64: make SPARSEMEM default and disable DISCONTIGMEM")
Reported-by: Guenter Roeck <linux@roeck-us.net>
Tested-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Mike Rapoport <rppt@linux.ibm.com>