add missing header

Change-Id: I3378c7c0bfdf5a6901e4ce8ecc4ae807a0d893eb

vcl/inc/unx/x11windowprovider.hxx

+/* -*- Mode: C++; tab-width: 4; indent-tabs-mode: nil; c-basic-offset: 4 -*- */
+/*
+ * This file is part of the LibreOffice project.
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/.
+ */
+
+#ifndef INCLUDED_VCL_UNX_X11WINDOWPROVIDER
+#define INCLUDED_VCL_UNX_X11WINDOWPROVIDER
+
+#include <prex.h>
+#include <postx.h>
+
+#include <vcl/dllapi.h>
+
+class VCL_PLUGIN_PUBLIC X11WindowProvider
+{
+public:
+    virtual ~X11WindowProvider();
+
+    virtual Window GetX11Window() = 0;
+};
+
+#endif
+
+/* vim:set shiftwidth=4 softtabstop=4 expandtab: */