made wxHeaderColumnBase dtor virtual to avoid gcc warnings

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@57164 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/headercol.h

@@ -67,6 +67,11 @@ public:
                    int flags = wxCOL_DEFAULT_FLAGS);
     */
 
+    // virtual dtor for the base class to avoid gcc warnings even though we
+    // don't normally delete the objects of this class via a pointer to
+    // wxHeaderColumnBase so it's not necessary, strictly speaking
+    virtual ~wxHeaderColumnBase() { }
+
 
     // setters and getters for various attributes
     // ------------------------------------------