mmc: rtsx: Explicitely include slab.h in rtsx_pci_sdmmc.c

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

drivers/mmc/host/rtsx_pci_sdmmc.c: In function 'sd_normal_rw':
drivers/mmc/host/rtsx_pci_sdmmc.c:448:2: error: implicit
declaration of function 'kzalloc'
[-Werror=implicit-function-declaration]
drivers/mmc/host/rtsx_pci_sdmmc.c:448:6: warning: assignment
makes pointer from integer without a cast [enabled by default]
drivers/mmc/host/rtsx_pci_sdmmc.c:472:2: error: implicit
declaration of function 'kfree'
[-Werror=implicit-function-declaration]

Signed-off-by: Wei WANG <wei_wang@realsil.com.cn>
Acked-by: Chris Ball <cjb@laptop.org>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>