Fixed discarded-qualifiers warning

Fixed discarded-qualifiers warning

by asigning the const pointer type to a non-const pointer type
the code was discarding the const qualifier. Added const key
word to fix warning.

Change-Id: Ided09959ac9406ed80f69606e9ed065d2498ebd2
Signed-off-by: George Nash <george.nash@intel.com>
Reviewed-on: https://gerrit.iotivity.org/gerrit/11527
Reviewed-by: Larry Sachs <larry.j.sachs@intel.com>
Tested-by: jenkins-iotivity <jenkins-iotivity@opendaylight.org>
Reviewed-by: jaehyun Cho <jaehyun3.cho@samsung.com>
Reviewed-by: Rick Bell <richard.s.bell@intel.com>