thread/Mutex: use using instead of making it a derived class

Prepare the transition to std::mutex.

src/lib/curl/Init.hxx

@@ -1,5 +1,5 @@
 /*
- * Copyright (C) 2008-2017 Max Kellermann <max.kellermann@gmail.com>
+ * Copyright 2008-2019 Max Kellermann <max.kellermann@gmail.com>
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
@@ -30,7 +30,8 @@
 #ifndef CURL_INIT_HXX
 #define CURL_INIT_HXX
 
-class Mutex;
+#include "thread/Mutex.hxx"
+
 class EventLoop;
 class CurlGlobal;