Fix a problem where the compiler would mix up two function declarations because the one wich has SGSubsystem in it's options list expected a const SGSubsystem, but it was called with a plain SGSubsystem

simgear/structure/event_mgr.cxx

@@ -71,12 +71,12 @@ SGEvent::SGEvent( const char* name,
 }
 
 SGEvent::SGEvent( const char* name,
-                  const SGSubsystem* subsystem,
+                  SGSubsystem* subsystem,
                   interval_type repeat_value,
                   interval_type initial_value )
     : _name(name),
       _callback(NULL),
-      _subsystem((SGSubsystem*)&subsystem),
+      _subsystem(subsystem),
       _repeat_value(repeat_value),
       _initial_value(initial_value),
       _cum_time(0),

simgear/structure/event_mgr.hxx

@@ -71,7 +71,7 @@ public:
              interval_type initial_value );
 
     SGEvent( const char* desc,
-             const SGSubsystem* subsystem,
+             SGSubsystem* subsystem,
              interval_type repeat_value,
              interval_type initial_value );
 
@@ -176,7 +176,7 @@ public:
      * -1 means run immediately.
      */
     inline void add( const char* name,
-                     const SGSubsystem* subsystem,
+                     SGSubsystem* subsystem,
                      interval_type repeat_value,
                      interval_type initial_value = -1 )
     {