drivers/gpio/vx855_gpio.c needs slab.h

alpha allmodconfig:

  drivers/gpio/vx855_gpio.c: In function 'vx855gpio_probe':
  drivers/gpio/vx855_gpio.c:233: error: implicit declaration of function 'kzalloc'
  drivers/gpio/vx855_gpio.c:233: warning: assignment makes pointer from integer without a cast

Cc: Grant Likely <grant.likely@secretlab.ca>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>