Make hidden declaration explicit.

Required to avoid shadowing warning by clang.

```
.../src/FWCore/ParameterSet/interface/ParameterDescriptionBase.h:99:18:
warning: 'edm::ParameterDescriptionBase::exists_' hides overloaded
virtual function [-Woverloaded-virtual]
     virtual bool exists_(ParameterSet const& pset, bool isTracked)
const = 0;
                 ^
.../src/FWCore/ParameterSet/interface/ParameterDescriptionNode.h:246:18:
note: hidden overloaded virtual function
'edm::ParameterDescriptionNode::exists_' declared here: different number
of parameters (1 vs 2)
    virtual bool exists_(ParameterSet const& pset) const = 0;
```