usb/ssb: Add missing #include <linux/slab.h>

m68k/allmodconfig:

drivers/usb/host/ssb-hcd.c: In function ‘ssb_hcd_probe’:
drivers/usb/host/ssb-hcd.c:170: error: implicit declaration of function ‘kzalloc’
drivers/usb/host/ssb-hcd.c:170: warning: assignment makes pointer from integer without a cast
drivers/usb/host/ssb-hcd.c:205: error: implicit declaration of function ‘kfree’

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>