SkTHash: add checksum include

Review URL: https://codereview.chromium.org/1030883006
diff --git a/src/core/SkTHash.h b/src/core/SkTHash.h
index 849c850..967b9be 100644
--- a/src/core/SkTHash.h
+++ b/src/core/SkTHash.h
@@ -1,6 +1,14 @@
+/*
+ * 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
 
+#include "SkChecksum.h"
 #include "SkTypes.h"
 #include "SkTemplates.h"