Revert wrong removal of proxy client sample build

Revert wrong removal of proxy client sample build

A DTLS related change has wrongly removed build for
proxy client sample.

Change-Id: I1f65c1cadf6a84a69691ad02bc1227793ca2fce7
Signed-off-by: Abhishek Sharma <ce.abhishek@samsung.com>
Reviewed-on: https://gerrit.iotivity.org/gerrit/13305
Tested-by: jenkins-iotivity <jenkins-iotivity@opendaylight.org>
Reviewed-by: Uze Choi <uzchoi@samsung.com>
(cherry picked from commit cf5614e1d93d537be20943136beb17b7182d87d2)
Reviewed-on: https://gerrit.iotivity.org/gerrit/15277
Reviewed-by: Dan Mihai <Daniel.Mihai@microsoft.com>