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

crypto/p224.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_P224_H_
 #define CRYPTO_P224_H_
 
 #include <string>
 
 #include "base/basictypes.h"
-#include "base/string_piece.h"
+#include "base/strings/string_piece.h"
 #include "crypto/crypto_export.h"
 
 namespace crypto {
 
 // P224 implements an elliptic curve group, commonly known as P224 and defined
 // in FIPS 186-3, section D.2.2.
 namespace p224 {
 
 // An element of the field (ℤ/pℤ) is represented with 8, 28-bit limbs in
 // little endian order.
@@ skipping 29 matching lines @@
 void CRYPTO_EXPORT Add(const Point& a, const Point& b, Point* out);
 
 // Negate calculates out = -a;
 void CRYPTO_EXPORT Negate(const Point& a, Point* out);
 
 }  // namespace p224
 
 }  // namespace crypto
 
 #endif  // CRYPTO_P224_H_