Reduce accessibility of method

This isn't used by anything else.

src/VisualStudio/Core/Def/Implementation/Workspace/WorkspaceFailureOutputPane.cs

@@ -99,7 +99,7 @@ private Guid GetActivePane(IVsOutputWindow outputWindow)
             return Guid.Empty;
         }
 
-        public void ActivatePane(IVsOutputWindow outputWindow, Guid paneGuid)
+        private void ActivatePane(IVsOutputWindow outputWindow, Guid paneGuid)
         {
             AssertIsForeground();