COMP: Fix -Wdelete-non-virtual-dtor in ctkDICOMDisplayedFieldGenerato…

…rAbstractRule

This commit fixes the following warning:

  /path/to/Slicer-SuperBuild/CTK/Libs/DICOM/Core/ctkDICOMDisplayedFieldGenerator.cpp: In destructor ‘virtual ctkDICOMDisplayedFieldGeneratorPrivate::~ctkDICOMDisplayedFieldGeneratorPrivate()’:
  /path/to/Slicer-SuperBuild/CTK/Libs/DICOM/Core/ctkDICOMDisplayedFieldGenerator.cpp:62:12: warning: deleting object of abstract class type ‘ctkDICOMDisplayedFieldGeneratorAbstractRule’ which has non-virtual destructor will cause undefined behaviour [-Wdelete-non-virtual-dtor]
       delete rule;
              ^

Libs/DICOM/Core/ctkDICOMDisplayedFieldGeneratorAbstractRule.h

@@ -42,6 +42,8 @@ class ctkDICOMDatabase;
 class CTK_DICOM_CORE_EXPORT ctkDICOMDisplayedFieldGeneratorAbstractRule
 {
 public:
+  virtual ~ctkDICOMDisplayedFieldGeneratorAbstractRule(){}
+
   /// Generate displayed fields for a certain instance based on its cached tags
   /// Each rule plugin has the chance to fill any field in the series, study, and patient fields.
   /// The way these generated fields will be used is defined by \sa mergeDisplayedFieldsForInstance