staging: comedi: mite: move #include <linux/slab.h>

The Comedi driver modules build fine if the inclusion of <linux/slab.h>
is removed from "mite.h", so remove it.  However, since "mite.c" calls
`kzalloc()`, include <linux/slab.h> there.

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