[C++] Allow instantiation of SubscriptionListner<void>.
[C++] Allow instantiation of SubscriptionListner<void>.

Constructor and members were made accidentaly private and class was not usable.

Change-Id: Ia7ddae3a5c1a09772cc32acc07eeef10ee2065fe

bmwcarit/joynr,bmwcarit/joynr,bmwcarit/joynr,bmwcarit/joynr,bmwcarit/joynr,bmwcarit/joynr,bmwcarit/joynr,bmwcarit/joynr,bmwcarit/joynr