Move SkTHash.h to include/private.

include/private/SkTHash.h

 /*
  * Copyright 2015 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef SkTHash_DEFINED
 #define SkTHash_DEFINED
 
@@ skipping 272 matching lines @@
 
 private:
     struct Traits {
         static const T& GetKey(const T& item) { return item; }
         static uint32_t Hash(const T& item) { return HashT(item); }
     };
     SkTHashTable<T, T, Traits> fTable;
 };
 
 #endif//SkTHash_DEFINED