staging: comedi: comedi_fc: remove 'extern' from function prototypes

The 'extern' is not needed in the prototypes for the exported functions
in the header. Remove it.

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>