COMP: Make GPUFunctorBase destructor virtual

The patch addresses the following warning:

ITK/Modules/Core/GPUCommon/include/itkGPUFunctorBase.h:43:3: warning:
'itk::Functor::GPUFunctorBase' has virtual functions but non-virtual
destructor [-Wnon-virtual-dtor]
  ~GPUFunctorBase() {

Change-Id: I55a2f002c036553f47bb8c468238f88e44e54147
Merge topic 'FixWaringNonvirtualDestructor'

36c9d33 COMP: Make GPUFunctorBase destructor virtual