Fix non-noise cpplint errors in //crypto.

crypto/hmac.cc

Index: crypto/hmac.cc
diff --git a/crypto/hmac.cc b/crypto/hmac.cc
index a9bb483f5b7f5708772d02583b8c5a89c8ee7dff..bf89e182d939c2ee45f001bbb3387518bf2d84d6 100644
--- a/crypto/hmac.cc
+++ b/crypto/hmac.cc
@@ -7,6 +7,7 @@
 #include <stddef.h>
 
 #include <algorithm>
+#include <string>
 
 #include "base/logging.h"
 #include "base/stl_util.h"