[mutter] startup: Fix build with 'startup_notification=false'

[Jonas Ådahl <jadahl src gnome org>]

commit 9f50b2cef270aa3e8ae4f3e9a0269d2de39ec9a2
Author: Christian Rauch <Rauch Christian gmx de>
Date:   Wed Dec 4 23:28:09 2019 +0000

    startup: Fix build with 'startup_notification=false'
    
    https://gitlab.gnome.org/GNOME/mutter/merge_requests/958

 src/x11/meta-startup-notification-x11.c | 6 ++++++
 1 file changed, 6 insertions(+)
---
diff --git a/src/x11/meta-startup-notification-x11.c b/src/x11/meta-startup-notification-x11.c
index 99fcf4561..045ee7768 100644
--- a/src/x11/meta-startup-notification-x11.c
+++ b/src/x11/meta-startup-notification-x11.c
@@ -292,16 +292,22 @@ gboolean
 meta_x11_startup_notification_handle_xevent (MetaX11Display *x11_display,
                                              XEvent         *xevent)
 {
+#ifdef HAVE_STARTUP_NOTIFICATION
   MetaX11StartupNotification *x11_sn = x11_display->startup_notification;
 
   if (!x11_sn)
     return FALSE;
 
   return sn_display_process_event (x11_sn->sn_display, xevent);
+#else
+  return FALSE;
+#endif
 }
 
+#ifdef HAVE_STARTUP_NOTIFICATION
 typedef void (* SetAppIdFunc) (SnLauncherContext *context,
                                const char        *app_id);
+#endif
 
 gchar *
 meta_x11_startup_notification_launch (MetaX11Display *x11_display,