Fix cloud client sample build failure
Fix cloud client sample build failure

1. Add reference to use sleep function

Change-Id: I0a6eec951f4c3b82ba77cf01a93c441fef7966d3
Signed-off-by: Jee Hyeok Kim <ecfb86b5d85d5430a7602d1e11f4f25d16714d36@samsung.com>
Reviewed-on: https://gerrit.iotivity.org/gerrit/5989
Tested-by: jenkins-iotivity <09cb29e8a2b473a2c978382eec13ee06fa017bda@opendaylight.org>

santais/iotivity_1.1,santais/iotivity_1.1,iotivity/iotivity,iotivity/iotivity,iotivity/iotivity,iotivity/iotivity,rzr/iotivity,rzr/iotivity,rzr/iotivity,santais/iotivity_1.1,rzr/iotivity,santais/iotivity_1.1,santais/iotivity_1.1,santais/iotivity_1.1,iotivity/iotivity,iotivity/iotivity,iotivity/iotivity,rzr/iotivity,rzr/iotivity,rzr/iotivity,santais/iotivity_1.1,iotivity/iotivity,santais/iotivity_1.1