[tracker/abderrahim/build-fix] tracker-store: fix build with vala master

[Abderrahim Kitouni <akitouni src gnome org>]

commit 5a5f586d106f5364b5a32b0c81fba00a331536e0
Author: Abderrahim Kitouni <akitouni gnome org>
Date:   Tue Apr 7 17:24:25 2020 +0100

    tracker-store: fix build with vala master

 src/tracker-store/tracker-dbus.vala | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/src/tracker-store/tracker-dbus.vala b/src/tracker-store/tracker-dbus.vala
index 82387d76e..dbbd775d6 100644
--- a/src/tracker-store/tracker-dbus.vala
+++ b/src/tracker-store/tracker-dbus.vala
@@ -122,7 +122,7 @@ public class Tracker.DBus {
                return true;
        }
 
-       static void name_owner_changed_cb (DBusConnection connection, string sender_name, string object_path, 
string interface_name, string signal_name, Variant parameters) {
+       static void name_owner_changed_cb (DBusConnection connection, string? sender_name, string 
object_path, string interface_name, string signal_name, Variant parameters) {
 
                unowned string name, old_owner, new_owner;
                parameters.get ("(&s&s&s)", out name, out old_owner, out new_owner);