security: Add SignatureSha256WithEcdsa

Change-Id: Ib3aa460d409c6d780b722e154ef2a620c510b16b
Refs: #1660
diff --git a/src/signature.hpp b/src/signature.hpp
index d678c8e..3546c4c 100644
--- a/src/signature.hpp
+++ b/src/signature.hpp
@@ -22,6 +22,8 @@
 #ifndef NDN_SIGNATURE_HPP
 #define NDN_SIGNATURE_HPP
 
+#include "encoding/tlv.hpp"
+
 namespace ndn {
 
 /**
@@ -41,8 +43,9 @@
   };
 
   enum {
-    Sha256 = 0,
-    Sha256WithRsa = 1
+    Sha256 = Tlv::DigestSha256,
+    Sha256WithRsa = Tlv::SignatureSha256WithRsa,
+    Sha256WithEcdsa = Tlv::SignatureSha256WithEcdsa
   };
 
   Signature()