[smuxi] Frontend-GNOME: bump copyright year

[Mirco M. M. Bauer <mmmbauer src gnome org>]

commit 44cae3baab7fb7310044186e8fbc83610dae86c0
Author: Mirco Bauer <meebey meebey net>
Date:   Fri Feb 12 20:22:07 2021 +0800

    Frontend-GNOME: bump copyright year

 src/Frontend-GNOME/AboutDialog.cs | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/src/Frontend-GNOME/AboutDialog.cs b/src/Frontend-GNOME/AboutDialog.cs
index 039474fd..0c32d27a 100644
--- a/src/Frontend-GNOME/AboutDialog.cs
+++ b/src/Frontend-GNOME/AboutDialog.cs
@@ -46,7 +46,7 @@ namespace Smuxi.Frontend.Gnome
             if (!Frontend.IsLocalEngine) {
                 Version += "\n Protocol: " + Frontend.EngineProtocolVersion;
             }
-            Copyright = "Copyright © 2005-2018 Mirco Bauer <meebey meebey net> and other contributors";
+            Copyright = "Copyright © 2005-2021 Mirco Bauer <meebey meebey net> and other contributors";
             Authors = new string[] {
                 "Mirco Bauer <meebey meebey net>",
                 "David Paleino <dapal debian org>",