[mutter] screen: Fix build

[Jasper St. Pierre <jstpierre src gnome org>]

commit d45080d32e41cadbdcace511eca6cc932128a49e
Author: Jasper St. Pierre <jstpierre mecheye net>
Date:   Sun Dec 28 19:36:29 2014 -0800

    screen: Fix build
    
    Forgot to squash again...

 src/core/screen.c |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)
---
diff --git a/src/core/screen.c b/src/core/screen.c
index 496e693..740a584 100644
--- a/src/core/screen.c
+++ b/src/core/screen.c
@@ -2889,7 +2889,7 @@ meta_screen_set_cm_selection (MetaScreen *screen)
 
   timestamp = meta_display_get_current_time_roundtrip (screen->display);
   g_snprintf (selection, sizeof (selection), "_NET_WM_CM_S%d", screen->number);
-  a = XInternAtom (xdisplay, selection, False);
+  a = XInternAtom (screen->display->xdisplay, selection, False);
   screen->wm_cm_selection_window = take_manager_selection (screen->display, screen->xroot, a, timestamp, 
TRUE);
 }