drivers/serial/mfd.c needs slab.h

alpha allmodconfig:

  drivers/serial/mfd.c:144: error: implicit declaration of function 'kzalloc'
  drivers/serial/mfd.c:144: warning: assignment makes pointer from integer without a cast

Cc: Greg Kroah-Hartman <gregkh@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>