[shotwell] Fix header bar for preferences dialog

[Jens Georg <jensgeorg src gnome org>]

commit 9db1e4db604a2e5791806d5c125a80779dbfa1ad
Author: Jens Georg <mail jensge org>
Date:   Sat Apr 22 09:30:54 2017 +0200

    Fix header bar for preferences dialog

 src/Dialogs.vala |    6 ++----
 1 files changed, 2 insertions(+), 4 deletions(-)
---
diff --git a/src/Dialogs.vala b/src/Dialogs.vala
index 5a89626..5ee1dd4 100644
--- a/src/Dialogs.vala
+++ b/src/Dialogs.vala
@@ -2323,10 +2323,8 @@ public class PreferencesDialog : Gtk.Dialog {
     private PreferencesDialog() {
         bool use_header;
         Gtk.Settings.get_default ().get ("gtk-dialogs-use-header", out use_header);
-        if (!use_header) {
-            Gtk.Widget null_titlebar = null;
-            set_titlebar (null_titlebar);
-        }
+        Object (use_header_bar: use_header ? 1 : 0);
+
         set_parent_window(AppWindow.get_instance().get_parent_window());
         set_transient_for(AppWindow.get_instance());
         delete_event.connect(on_delete);