IrtGeo: mark destructor as virtual (#462)

```
src/services/geometry/rich/RichGeo_service.cc:73:18: warning: delete called on 'rich::IrtGeo' that is abstract but has non-virtual destructor [-Wdelete-abstract-non-virtual-dtor]
    if(m_irtGeo) delete m_irtGeo;
                 ^
```

src/services/geometry/rich/richgeo/IrtGeo.h

@@ -32,7 +32,7 @@ namespace rich {
       IrtGeo(std::string detName_, dd4hep::Detector *det_, bool verbose_=false);
       // alternate constructor: use compact file for DD4hep geometry (backward compatibility)
       IrtGeo(std::string detName_, std::string compactFile_="", bool verbose_=false);
-      ~IrtGeo(); 
+      virtual ~IrtGeo();
 
       // access the full IRT geometry
       CherenkovDetectorCollection *GetIrtDetectorCollection() { return m_irtDetectorCollection; }