Add WB_PROTECTED to mutexes

mutex_mark is (basically) NULL, so we don't have any references to mark.
This means we should safely be able to mark Mutex as WB_PROTECTED
without changing anything else.

thread_sync.c

@@ -125,7 +125,7 @@ mutex_memsize(const void *ptr)
 static const rb_data_type_t mutex_data_type = {
     "mutex",
     {mutex_mark, mutex_free, mutex_memsize,},
-    0, 0, RUBY_TYPED_FREE_IMMEDIATELY
+    0, 0, RUBY_TYPED_WB_PROTECTED | RUBY_TYPED_FREE_IMMEDIATELY
 };
 
 static rb_mutex_t *