g_icon_serialize is only in glib >= 2.38

Change-Id: I824ebd9083bd702e90f577471248be43707cc811
diff --git a/vcl/unx/gtk/glomenu.cxx b/vcl/unx/gtk/glomenu.cxx
index ffa43e5..aab6857 100644
--- a/vcl/unx/gtk/glomenu.cxx
+++ b/vcl/unx/gtk/glomenu.cxx
@@ -238,7 +238,13 @@ g_lo_menu_set_icon (GLOMenu     *menu,
    GVariant *value;

    if (icon != nullptr)
    {
#if GLIB_CHECK_VERSION(2,38,0)
        value = g_icon_serialize (const_cast<GIcon*>(icon));
#else
        value = nullptr;
#endif
    }
    else
        value = nullptr;