mfd: Explicitely include slab.h to rtsx

mfd: Explicitely include slab.h to rtsx

This fixes the following build error on some architectures (parisc at least):

drivers/mfd/rtsx_pcr.c: In function 'rtsx_pci_init_chip':
drivers/mfd/rtsx_pcr.c:985:2: error: implicit declaration of function 'kcalloc'
[-Werror=implicit-function-declaration]
drivers/mfd/rtsx_pcr.c:985:13: warning: assignment makes pointer from integer
without a cast [enabled by default]

Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>