coap-http-proxy: Fix coap headers path

coap-http-proxy: Fix coap headers path

It failed to build on Tizen:2.3.1:Wearable

Change-Id: I1069ab17f11a946605093454b89205a6c208c902
Origin: https://gerrit.iotivity.org/gerrit/#/c/12061/
Signed-off-by: Philippe Coval <philippe.coval@osg.samsung.com>
Reviewed-on: https://gerrit.iotivity.org/gerrit/12061
Tested-by: jenkins-iotivity <jenkins-iotivity@opendaylight.org>
Reviewed-by: Ashwini  Kumar <k.ashwini@samsung.com>
Reviewed-by: jihwan seo <jihwan.seo@samsung.com>
Reviewed-by: Habib Virji <habib.virji@samsung.com>
Reviewed-by: Abhishek Sharma <ce.abhishek@samsung.com>
Reviewed-by: Ziran Sun <ziran.sun@samsung.com>