let have FormulaGroupAreaListener a virtual dtor

Change-Id: I1f5e246d68971124ef21e9c2d0db80499928998b

sc/inc/grouparealistener.hxx

@@ -36,6 +36,8 @@ public:
     FormulaGroupAreaListener(
         const ScRange& rRange, ScFormulaCell** ppTopCell, SCROW nGroupLen, bool bStartFixed, bool bEndFixed );
 
+    virtual ~FormulaGroupAreaListener();
+
     ScRange getListeningRange() const;
 
     virtual void Notify( const SfxHint& rHint ) SAL_OVERRIDE;

sc/source/core/tool/grouparealistener.cxx

@@ -80,6 +80,10 @@ FormulaGroupAreaListener::FormulaGroupAreaListener(
     assert(mppTopCell); // This can't be NULL.
 }
 
+FormulaGroupAreaListener::~FormulaGroupAreaListener()
+{
+}
+
 ScRange FormulaGroupAreaListener::getListeningRange() const
 {
     ScRange aRet = maRange;