Add virtual dtor to wxCustomBackgroundWindowBase.

Suppress g++ warnings about a class with virtual methods but non-virtual dtor
by making the dtor virtual even if we don't need it to be virtual in this
class.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@69930 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/custombgwin.h

@@ -22,6 +22,9 @@ public:
     // Trivial default ctor.
     wxCustomBackgroundWindowBase() { }
 
+    // Also a trivial but virtual -- to suppress g++ warnings -- dtor.
+    virtual ~wxCustomBackgroundWindowBase() { }
+
     // Use the given bitmap to tile the background of this window. This bitmap
     // will show through any transparent children.
     //