mixer: protect the mixer struct with a mutex

In some rare cases, there was a race condition between the output
thread and the main thread: when you disable/enable an output device
in the main thread, this caused a crash in the output thread.  Protect
the whole mixer struct with a GMutex to prevent that.

src/mixer_api.c

@@ -19,8 +19,6 @@
 
 #include "mixer_api.h"
 
-#include <glib.h>
-
 #undef G_LOG_DOMAIN
 #define G_LOG_DOMAIN "mixer"
 
@@ -28,4 +26,5 @@ void
 mixer_init(struct mixer *mixer, const struct mixer_plugin *plugin)
 {
 	mixer->plugin = plugin;
+	mixer->mutex = g_mutex_new();
 }