Include cstdlib in map_test to make sure std::rand is available when
building with libc++.
diff --git a/test/map_test.cc b/test/map_test.cc
index 58399c1..5eccf8d 100644
--- a/test/map_test.cc
+++ b/test/map_test.cc
@@ -1,5 +1,6 @@
 #include "benchmark/benchmark.h"
 
+#include <cstdlib>
 #include <map>
 
 namespace {