Fix build error with libc++
diff --git a/test/complexity_test.cc b/test/complexity_test.cc
index b8cd440..6e6ae3c 100644
--- a/test/complexity_test.cc
+++ b/test/complexity_test.cc
@@ -1,6 +1,7 @@
 
 #include "benchmark/benchmark_api.h"
 
+#include <cstdlib>
 #include <string>
 #include <vector>
 #include <map>