[gdm] Fix missing class cast

[Robert Ancell <rancell src gnome org>]

commit 779bf2440e5ac3733b831e1c1422246b171bd52a
Author: Robert Ancell <robert ancell canonical com>
Date:   Mon Dec 4 12:20:40 2017 +1300

    Fix missing class cast

 daemon/gdm-legacy-display.c |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)
---
diff --git a/daemon/gdm-legacy-display.c b/daemon/gdm-legacy-display.c
index d9c4336..cd6259b 100644
--- a/daemon/gdm-legacy-display.c
+++ b/daemon/gdm-legacy-display.c
@@ -267,7 +267,7 @@ on_display_status_changed (GdmLegacyDisplay *self)
 {
         int status;
 
-        status = gdm_display_get_status (self);
+        status = gdm_display_get_status (GDM_DISPLAY (self));
 
         switch (status) {
             case GDM_DISPLAY_UNMANAGED: