gui: Restore broken logo on mobile

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/3238

gui/default/index.html

@@ -34,7 +34,7 @@
     <div class="container">
       <span class="navbar-brand">
         <img class="logo hidden-xs" src="assets/img/logo-horizontal.svg" height="32" width="117"/>
-        <img class="logo hidden visible-xs" src="assets/img/favicon.png" height="32"/>
+        <img class="logo hidden visible-xs" src="assets/img/favicon-default.png" height="32"/>
       </span>
       <p class="navbar-text hidden-xs" ng-class="{'hidden-sm':upgradeInfo && upgradeInfo.newer}">{{thisDeviceName()}}</p>
       <ul class="nav navbar-nav navbar-right">