Make wxGenericCredentialEntryDialog non-copyable

No real changes, just a bit of extra safety.

include/wx/generic/creddlgg.h

@@ -45,6 +45,8 @@ private:
     void Init(const wxString& message,
         const wxString& user,
         const wxString& password);
+
+    wxDECLARE_NO_COPY_CLASS(wxGenericCredentialEntryDialog);
 };
 
 // Add this typedef as long as the generic version is the only one available