Add MinGW std threads include to music/win32_m.cpp

Fix uses of std::lock_guard

src/music/win32_m.cpp

@@ -20,6 +20,10 @@
 #include "midi.h"
 #include "../base_media_base.h"
 #include <mutex>
+#if defined(__MINGW32__)
+#include "../3rdparty/mingw-std-threads/mingw.mutex.h"
+#include "../3rdparty/mingw-std-threads/mingw.condition_variable.h"
+#endif
 
 #include "../safeguards.h"
 
@@ -328,7 +332,7 @@ void MusicDriver_Win32::PlaySong(const MusicSongInfo &song)
 	if (!new_song.LoadSong(song)) return;
 	DEBUG(driver, 2, "Win32-MIDI: PlaySong: Loaded song");
 
-	std::lock_guard mutex_lock(_midi.lock);
+	std::lock_guard<std::mutex> mutex_lock(_midi.lock);
 
 	_midi.next_file.MoveFrom(new_song);
 	_midi.next_segment.start = song.override_start;
@@ -348,7 +352,7 @@ void MusicDriver_Win32::PlaySong(const MusicSongInfo &song)
 void MusicDriver_Win32::StopSong()
 {
 	DEBUG(driver, 2, "Win32-MIDI: StopSong: entry");
-	std::lock_guard mutex_lock(_midi.lock);
+	std::lock_guard<std::mutex> mutex_lock(_midi.lock);
 	DEBUG(driver, 2, "Win32-MIDI: StopSong: setting flag");
 	_midi.do_stop = true;
 }
@@ -360,7 +364,7 @@ bool MusicDriver_Win32::IsSongPlaying()
 
 void MusicDriver_Win32::SetVolume(byte vol)
 {
-	std::lock_guard mutex_lock(_midi.lock);
+	std::lock_guard<std::mutex> mutex_lock(_midi.lock);
 	_midi.new_volume = vol;
 }
 
@@ -402,7 +406,7 @@ const char *MusicDriver_Win32::Start(const char * const *parm)
 
 void MusicDriver_Win32::Stop()
 {
-	std::lock_guard mutex_lock(_midi.lock);
+	std::lock_guard<std::mutex> mutex_lock(_midi.lock);
 
 	if (_midi.timer_id) {
 		timeKillEvent(_midi.timer_id);