Update the remaining include paths of base/string_piece.h to its new location.

crypto/openpgp_symmetric_encryption.h

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef CRYPTO_OPENPGP_SYMMETRIC_ENCRYPTION_H_
 #define CRYPTO_OPENPGP_SYMMETRIC_ENCRYPTION_H_
 
 #include <string>
 
-#include "base/string_piece.h"
+#include "base/strings/string_piece.h"
 #include "crypto/crypto_export.h"
 
 namespace crypto {
 
 // OpenPGPSymmetricEncrytion implements enough of RFC 4880 to read and write
 // uncompressed, symmetrically encrypted data. You can create ciphertext
 // compatable with this code from the command line with:
 //    gpg --compress-algo=NONE --cipher-algo=AES -c
 //
 // Likewise, the output of this can be decrypted on the command line with:
@@ skipping 15 matching lines @@
                         std::string *out);
 
   static std::string Encrypt(base::StringPiece plaintext,
                              base::StringPiece passphrase);
 };
 
 }  // namespace crypto
 
 #endif  // CRYPTO_OPENPGP_SYMMETRIC_ENCRYPTION_H_