staging: comedi: ke_counter: expose clock source options to user space

As pointed out by Ian Abbott, the INSN_CONFIG_SET_CLOCK_SRC options should
be exposed in the user header comedi.h.

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>