[gtk] Fix compile warning

[Benjamin Otte <otte src gnome org>]

commit 551ced9ae44d11bd24064e6829a8900eb1eaca65
Author: Benjamin Otte <otte redhat com>
Date:   Sat Mar 23 03:08:03 2019 +0100

    Fix compile warning

 gtk/gtkmain.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)
---
diff --git a/gtk/gtkmain.c b/gtk/gtkmain.c
index 193bd10b3c..d2dc80e3ac 100644
--- a/gtk/gtkmain.c
+++ b/gtk/gtkmain.c
@@ -1432,8 +1432,7 @@ is_or_contains (gboolean      enter,
       break;
     case GDK_NOTIFY_UNKNOWN:
     default:
-      g_warning ("Unknown focus change detail");
-      return;
+      g_return_val_if_reached (FALSE);
     }
 
   return is || contains;