fix after cleanup

fixes following compilation error:
libcnipol.so: undefined reference to `ChannelData::~ChannelData()'
libcnipol.so: undefined reference to `typeinfo for ChannelData'
libcnipol.so: undefined reference to `vtable for ChannelData'
collect2: ld returned 1 exit status

which basically means that virtual classes must have a defined virtual
destructor