greybus: operation: fix missing symbol exports

Add missing EXPORT_SYMBOL_GPL for gb_operation_response_alloc,
gb_operation_result, gb_operation_get, gb_operation_request_send and
gb_operation_cancel, which are all supposed to be accessible from
protocol handlers.

Signed-off-by: Johan Hovold <johan@hovoldconsulting.com>
Reviewed-by: Alex Elder <elder@linaro.org>
Signed-off-by: Greg Kroah-Hartman <greg@kroah.com>