closeCallback is already lockable, initialize it as such

Luckily we are allocating structs as clean memory and
PTHREAD_MUTEX_INITIALIZER is "{ 0 }", so nothing happened, but it should
still be created as lockable object.
Signed-off-by: NMartin Kletzander <mkletzan@redhat.com>

src/datatypes.c

 /*
  * datatypes.c: management of structs for public data types
  *
- * Copyright (C) 2006-2014 Red Hat, Inc.
+ * Copyright (C) 2006-2015 Red Hat, Inc.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Lesser General Public
@@ -113,7 +113,7 @@ virGetConnect(void)
     if (!(ret = virObjectNew(virConnectClass)))
         return NULL;
 
-    if (!(ret->closeCallback = virObjectNew(virConnectCloseCallbackDataClass)))
+    if (!(ret->closeCallback = virObjectLockableNew(virConnectCloseCallbackDataClass)))
         goto error;
 
     if (virMutexInit(&ret->lock) < 0)