[gtk] Fix a compiler warning

[Matthias Clasen <matthiasc src gnome org>]

commit 77ff83cdd14352015514c14f4fab33e164b83977
Author: Matthias Clasen <mclasen redhat com>
Date:   Sat Mar 23 00:13:15 2019 -0400

    Fix a compiler warning

 gtk/gtkmain.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)
---
diff --git a/gtk/gtkmain.c b/gtk/gtkmain.c
index 8becf17e91..22513d95c3 100644
--- a/gtk/gtkmain.c
+++ b/gtk/gtkmain.c
@@ -1432,7 +1432,8 @@ is_or_contains (gboolean      enter,
       break;
     case GDK_NOTIFY_UNKNOWN:
     default:
-      g_return_val_if_reached (FALSE);
+      g_warning ("Unknown focus change detail");
+      break;
     }
 
   return is || contains;