Fixed build issue by generating ~DiscoveryTask code

Fixed build issue by generating ~DiscoveryTask code

If declared as default (and not defined), this function will be inlined,
but in some cases (using shared libraries?) inlining is not appropriate,
so it's preferred to make a regular declaration and still use the default
capabilities for definition.

Problem was observed while building using tizen-sdk :

    libresource_hosting.so: undefined reference to \
    `OIC::Service::RCSDiscoveryManager::DiscoveryTask::~DiscoveryTask()'

This message apeared while building RHSampleApp using tizen-sdk 2.4.0 Rev4
with imported libs (built with gbs from Tizen:2.4:Mobile repo)

Used toolchain for Tizen:2.4:Mobile is arm-linux-gnueabi-g++ (GCC) 4.9.2

Change-Id: I708812404b01c35f09e8aa68afad962c7d6e0df3
Origin: https://github.com/TizenTeam/iotivity/tree/sandbox/pcoval/for-upstream
Signed-off-by: Philippe Coval <philippe.coval@osg.samsung.com>
Reviewed-on: https://gerrit.iotivity.org/gerrit/5445
Reviewed-by: Robert Spielmann <spielmann@kellendonk.de>
Tested-by: jenkins-iotivity <jenkins-iotivity@opendaylight.org>
Reviewed-by: Habib Virji <habib.virji@samsung.com>