staging: comedi: ni_660x: remove #include "comedi_fc.h"

"ni_660x.c" doesn't use anything from "comedi_fc.h".

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>