fix vm_events_fold_cpu() build breakage

2.6.20-rc1 does not build properly if CONFIG_VM_EVENT_COUNTERS is set
and CONFIG_HOTPLUG is unset:

  CC      init/version.o
  LD      init/built-in.o
  LD      .tmp_vmlinux1
mm/built-in.o: In function `page_alloc_cpu_notify':
page_alloc.c:(.text+0x56eb): undefined reference to `vm_events_fold_cpu'
make: *** [.tmp_vmlinux1] Error 1

[akpm@osdl.org: cleanup]
Signed-off-by: NMagnus Damm <magnus@valinux.co.jp>
Cc: Christoph Lameter <clameter@engr.sgi.com>
Signed-off-by: NAndrew Morton <akpm@osdl.org>
Signed-off-by: NLinus Torvalds <torvalds@osdl.org>