display: Don't include sn.h from display-private.h

It has some annoying macro requirement to not nag about API stability;
try to limit its exposure.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1861>

src/core/display-private.h

@@ -31,10 +31,6 @@
 #include <X11/extensions/sync.h>
 #include <X11/Xlib.h>
 
-#ifdef HAVE_STARTUP_NOTIFICATION
-#include <libsn/sn.h>
-#endif
-
 #include "clutter/clutter.h"
 #include "core/keybindings-private.h"
 #include "core/meta-gesture-tracker-private.h"

src/tests/test-utils.c

@@ -23,6 +23,7 @@
 
 #include <gio/gio.h>
 #include <string.h>
+#include <X11/Xlib-xcb.h>
 
 #include "core/display-private.h"
 #include "core/window-private.h"

src/x11/meta-startup-notification-x11.c

@@ -20,6 +20,7 @@
 #include "meta-startup-notification-x11.h"
 
 #include <gio/gdesktopappinfo.h>
+#include <libsn/sn.h>
 
 #include "core/display-private.h"
 #include "core/startup-notification-private.h"