staging: comedi: addi_apci_3120: remove unnecessary include

The <linux/sched.h> header is not needed by this driver. Remove the
include.

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