Make SGSubsystemGroup::Member copy constructor private.

The copy constructor wouldn't make any sense and would confuse ownership of
the subsystem pointer. Problem noted by John Denker.

simgear/structure/subsystem_mgr.cxx

@@ -290,9 +290,9 @@ SGSubsystemGroup::Member::Member ()
 {
 }
 
+// This shouldn't be called due to subsystem pointer ownership issues.
 SGSubsystemGroup::Member::Member (const Member &)
 {
-    Member();
 }
 
 SGSubsystemGroup::Member::~Member ()

simgear/structure/subsystem_mgr.hxx

@@ -317,10 +317,12 @@ public:
 
 private:
 
-    struct Member {
+    class Member {
 
-        Member ();
+    private:
         Member (const Member &member);
+    public:
+        Member ();
         virtual ~Member ();
 
         virtual void update (double delta_time_sec);