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;
 

src/lib/smbclient/Mutex.cxx

@@ -18,6 +18,5 @@
  */
 
 #include "Mutex.hxx"
-#include "thread/Mutex.hxx"
 
 Mutex smbclient_mutex;

src/lib/smbclient/Mutex.hxx

@@ -20,7 +20,7 @@
 #ifndef MPD_SMBCLIENT_MUTEX_HXX
 #define MPD_SMBCLIENT_MUTEX_HXX
 
-class Mutex;
+#include "thread/Mutex.hxx"
 
 /**
  * Since libsmbclient is not thread-safe, this mutex must be locked