fix compile when spinlocks aren't available

include/vlc_threads_funcs.h

@@ -626,7 +626,7 @@ typedef vlc_mutex_t vlc_spinlock_t;
 
 static inline int vlc_spin_init (vlc_spinlock_t *spin)
 {
-    return __vlc_mutex_init (NULL, spin);
+    return __vlc_mutex_init (spin);
 }
 
 # define vlc_spin_lock    vlc_mutex_lock