drivers/staging/comedi/drivers/das16.c: needs slab.h

alpha allmodconfig:

drivers/staging/comedi/drivers/das16.c: In function 'das16_detach':
drivers/staging/comedi/drivers/das16.c:1234: error: implicit declaration of function 'kfree'

Cc: Ian Abbott <abbotti@mev.co.uk>
Reviewed-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>