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

Prepare the transition to std::mutex.

src/input/InputPlugin.hxx

@@ -21,10 +21,10 @@
 #define MPD_INPUT_PLUGIN_HXX
 
 #include "Ptr.hxx"
+#include "thread/Mutex.hxx"
 #include "util/Compiler.h"
 
 struct ConfigBlock;
-class Mutex;
 class EventLoop;
 class RemoteTagScanner;
 class RemoteTagHandler;

src/input/LocalOpen.hxx

@@ -21,9 +21,9 @@
 #define MPD_INPUT_LOCAL_OPEN_HXX
 
 #include "Ptr.hxx"
+#include "thread/Mutex.hxx"
 
 class Path;
-class Mutex;
 
 /**
  * Open a "local" file.  This is a wrapper for the input plugins

src/input/plugins/ArchiveInputPlugin.hxx

@@ -21,9 +21,9 @@
 #define MPD_INPUT_ARCHIVE_HXX
 
 #include "input/Ptr.hxx"
+#include "thread/Mutex.hxx"
 
 class Path;
-class Mutex;
 
 InputStreamPtr
 OpenArchiveInputStream(Path path, Mutex &mutex);

src/input/plugins/CurlInputPlugin.hxx

@@ -21,12 +21,11 @@
 #define MPD_INPUT_CURL_HXX
 
 #include "input/Ptr.hxx"
+#include "thread/Mutex.hxx"
 
 #include <string>
 #include <map>
 
-class Mutex;
-
 extern const struct InputPlugin input_plugin_curl;
 
 /**

src/input/plugins/FileInputPlugin.hxx

@@ -21,9 +21,9 @@
 #define MPD_INPUT_FILE_HXX
 
 #include "input/Ptr.hxx"
+#include "thread/Mutex.hxx"
 
 class Path;
-class Mutex;
 
 InputStreamPtr
 OpenFileInputStream(Path path, Mutex &mutex);