[PATCH] vmstat: export all_vm_events()

Add missing EXPORT_SYMBOL for all_vm_events(). Git commit
f8891e5e caused this:

  Building modules, stage 2.
  MODPOST
WARNING: "all_vm_events" [arch/s390/appldata/appldata_mem.ko] undefined!
  CC      arch/s390/appldata/appldata_mem.mod.o

Cc: Christoph Lameter <christoph@lameter.com>
Cc: Gerald Schaefer <geraldsc@de.ibm.com>
Cc: Martin Schwidefsky <schwidefsky@de.ibm.com>
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>