seafile-client: add greizgh as maintainer due to their own request

pkgs/applications/networking/seafile-client/default.nix

···
     description = "Desktop client for Seafile, the Next-generation Open Source Cloud Storage";
     license = licenses.asl20;
     platforms = platforms.linux;
-    maintainers = with maintainers; [ schmittlauch ];
+    maintainers = with maintainers; [ schmittlauch greizgh ];
   };
 }