gtktiledviewer: Fix crash on copy.

Change-Id: I1afc6278f6853d5e45a305b4f731cb6af0e00cc1
Reviewed-on: https://gerrit.libreoffice.org/72955
Tested-by: Jenkins
Reviewed-by: Michael Meeks <michael.meeks@collabora.com>

libreofficekit/source/gtk/lokdocview.cxx

@@ -1452,6 +1452,7 @@ callback (gpointer pData)
     case LOK_CALLBACK_INVALIDATE_HEADER:
         g_signal_emit(pCallback->m_pDocView, doc_view_signals[INVALIDATE_HEADER], 0, pCallback->m_aPayload.c_str());
         break;
+    case LOK_CALLBACK_CLIPBOARD_CHANGED:
     case LOK_CALLBACK_CONTEXT_CHANGED:
         break; // TODO
     default: