Staging: iio: pull in slab.h for kmalloc funcs

These drivers use kzalloc() but don't include slab.h.  They currently
build though because the spi.h header will pull in slab.h for us.  But
rather than rely on that behavior forever, include slab.h explicitly.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Acked-by: Jonathan Cameron <jic23@cam.ac.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>