Add virtual dtor to header renderer classes to avoid g++ warnings.

g++ warns about class with virtual functions having non-virtual dtor so make
the dtor of wxGridCornerHeaderRenderer virtual even if it's not really needed.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@61933 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/generic/grid.h

@@ -295,6 +295,11 @@ public:
     virtual void DrawBorder(const wxGrid& grid,
                             wxDC& dc,
                             wxRect& rect) const = 0;
+
+    // make the dtor of a class with virtual functions virtual to avoid g++
+    // warnings, even though this class is not supposed to be used
+    // polymorphically
+    virtual ~wxGridCornerHeaderRenderer() { }
 };