PCI: fix ats compile failure

I get this compile failure on parisc:

drivers/pci/ats.c: In function 'ats_alloc_one':
drivers/pci/ats.c:29: error: implicit declaration of function 'kzalloc'
drivers/pci/ats.c:29: warning: assignment makes pointer from integer without a cast
drivers/pci/ats.c: In function 'ats_free_one':
drivers/pci/ats.c:45: error: implicit declaration of function 'kfree'

Because ats.c is missing linux/slab.h as an include.  This patch fixes it

Signed-off-by: James Bottomley <JBottomley@Parallels.com>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>