staging: comedi: mite.h: needs <linux/slab.h>

As reported by the kbuild test robot:

Commit: 0bdab509
staging: comedi: usee comedi_alloc_devpriv

Removed the include of <linux/slab.h> from many of the comedi drivers.
Unfortunately, this header uses kfree() in an inline function.

Instead of adding <linux/slab.h> to each of the drivers that use this
header, include slab.h here.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Reported-by: kbuild test robot <fengguang.wu@intel.com>
Cc: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>