lok: We changed the (unstable) ABI, but let's allow the older C++ API.

Helps building the Online before the appropriate patch gets merged.

Change-Id: I9983d546bb0cb7fb472806622a0a10e3a4d3ab87
Reviewed-on: https://gerrit.libreoffice.org/71665Reviewed-by: Michael Meeks <michael.meeks@collabora.com>
Tested-by: Michael Meeks <michael.meeks@collabora.com>

include/LibreOfficeKit/LibreOfficeKit.hxx

@@ -188,7 +188,7 @@ public:
      *
      * @param nWindowid
      */
-    void postWindow(unsigned nWindowId, int nAction, const char* pData)
+    void postWindow(unsigned nWindowId, int nAction, const char* pData = nullptr)
     {
         return mpDoc->pClass->postWindow(mpDoc, nWindowId, nAction, pData);
     }

libreofficekit/qa/tilebench/tilebench.cxx

@@ -286,7 +286,7 @@ static void testDialog( Document *pDocument, const char *uno_cmd )
     }
 
     aTimes.emplace_back("post close dialog");
-    pDocument->postWindow(nDialogId, LOK_WINDOW_CLOSE, nullptr);
+    pDocument->postWindow(nDialogId, LOK_WINDOW_CLOSE);
     aTimes.emplace_back();
 
     pDocument->destroyView(view);