G_MENU_ATTRIBUTE_ICON is only in glib >= 2.38

Change-Id: I7761ac05fbc9be39a6982ba66054c75a331816be
diff --git a/vcl/unx/gtk/glomenu.cxx b/vcl/unx/gtk/glomenu.cxx
index aab6857..f91a469 100644
--- a/vcl/unx/gtk/glomenu.cxx
+++ b/vcl/unx/gtk/glomenu.cxx
@@ -248,6 +248,10 @@ g_lo_menu_set_icon (GLOMenu     *menu,
    else
        value = nullptr;

#ifndef G_MENU_ATTRIBUTE_ICON
#    define G_MENU_ATTRIBUTE_ICON "icon"
#endif

    g_lo_menu_set_attribute_value (menu, position, G_MENU_ATTRIBUTE_ICON, value);
}