mixer: add missing include

src/lib/mixer/mixer_simple.cpp

@@ -39,6 +39,7 @@
 
 #include "mixer.h"
 #include <stdio.h>
+#include <stdlib.h>
 
 #define debug(fmt, args...)	do { } while(0)
 //#define debug(fmt, args...)	do { printf("[mixer] " fmt "\n", ##args); } while(0)