added empty virtual dtors to silence gcc warnings

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@41718 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

utils/HelpGen/src/srcparser.h

@@ -217,6 +217,8 @@ public:
     virtual void VisitParameter( spParameter& WXUNUSED(param) ) {}
 
     virtual void VisitCustomContext( spContext& WXUNUSED(ctx) ) {}
+
+    virtual ~spVisitor() { }
 };
 
 // stores one section of comments,
@@ -712,6 +714,8 @@ class SourceParserPlugin
 public:
     virtual bool CanUnderstandContext( char* cur, char* end, spContext* pOuttterCtx ) = 0;
     virtual void ParseContext( char* start, char*& cur, char* end, spContext* pOuttterCtx ) = 0;
+
+    virtual ~SourceParserPlugin() { }
 };
 
 // abstract interface for source parsers