Fix a compile error with some compilers introduced in r76812.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_3_0_BRANCH@76884 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

samples/treectrl/treetest.h

@@ -45,7 +45,7 @@ public:
     MyTreeItemData(const wxString& desc) : m_desc(desc) { }
 
     void ShowInfo(wxTreeCtrl *tree);
-    const wxChar *GetDesc() const { return m_desc.c_str(); }
+    wxString const& GetDesc() const { return m_desc; }
 
 private:
     wxString m_desc;