cpp: Include malloc/free header

include/lc3_cpp.h

diff --git a/include/lc3_cpp.h b/include/lc3_cpp.h
index 1aac6d8..acd3d0b 100644
--- a/include/lc3_cpp.h
+++ b/include/lc3_cpp.h
@@ -26,6 +26,7 @@
 #include <cassert>
 #include <memory>
 #include <vector>
+#include <stdlib.h>
 
 #include "lc3.h"