MAINT-5232: Correct forward declaration of LLSingleton_manage_master.

The forward declaration said it was a 'friend class', whereas the actual
definition is a struct. MSVC dislikes that.

indra/llcommon/llsingleton.h

@@ -67,7 +67,7 @@ class LLSingletonBase: private boost::noncopyable
     void add_master();
     void remove_master();
     // with a little help from our friends.
-    template <class T> friend class LLSingleton_manage_master;
+    template <class T> friend struct LLSingleton_manage_master;
 
     // Maintain a stack of the LLSingleton subclass instance currently being
     // initialized. We use this to notice direct dependencies: we want to know