Fix failed self test under Clang (GH #533)

This may cause GH #300, "Clang 3.9 and missing member definitions for template classes" or GH #294, "Fix clang warnings about undefined variable templates in pkcspad.h" to resurface. Man I hope not...

hashfwd.h

@@ -0,0 +1,35 @@
+// hashfwd.h - written and placed in the public domain by Jeffrey Walton
+
+/// \file hashfwd.h
+/// \brief Forward declarations for hash functions used in signature encoding methods
+
+#ifndef CRYPTOPP_HASHFWD_H
+#define CRYPTOPP_HASHFWD_H
+
+#include "config.h"
+
+NAMESPACE_BEGIN(CryptoPP)
+
+class SHA1;
+class SHA224;
+class SHA256;
+class SHA384;
+class SHA512;
+
+// http://github.com/weidai11/cryptopp/issues/517
+class SHA3_256;
+class SHA3_384;
+class SHA3_512;
+
+class Tiger;
+class RIPEMD128;
+class RIPEMD160;
+
+namespace Weak1 {
+  class MD2;
+  class MD5;
+}
+
+NAMESPACE_END
+
+#endif  // CRYPTOPP_HASHFWD_H