Deprecate vlc_object_alive()

The biggest landmark of thread-unsafety.

include/vlc_objects.h

@@ -87,7 +87,7 @@ VLC_API void vlc_object_kill( vlc_object_t * ) VLC_DEPRECATED;
 #define vlc_object_kill(a) \
     vlc_object_kill( VLC_OBJECT(a) )
 
-VLC_USED
+VLC_USED VLC_DEPRECATED
 static inline bool vlc_object_alive (const vlc_object_t *obj)
 {
     barrier ();