Fix memory leak in wxPanel::SetBackgroundBirmap() in wxMSW.

The brush allocated for the bitmap was never freed, do free it in dtor.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@68299 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/msw/panel.h

@@ -34,6 +34,8 @@ public:
         Create(parent, winid, pos, size, style, name);
     }
 
+    virtual ~wxPanel();
+
     // This is overridden for MSW to return true for all panels that are child
     // of a window with themed background (such as wxNotebook) which should
     // show through the child panels.