security: rename SignatureSha256 to DigestSha256

Change-Id: Idabb95222d86e2541372593698ed9754156a5df2
diff --git a/src/security/validator-config.hpp b/src/security/validator-config.hpp
index a55797a..f768efc 100644
--- a/src/security/validator-config.hpp
+++ b/src/security/validator-config.hpp
@@ -268,7 +268,7 @@
 {
   if (signature.getType() == Signature::Sha256)
     {
-      SignatureSha256 sigSha256(signature);
+      DigestSha256 sigSha256(signature);
 
       if (verifySignature(packet, sigSha256))
         return onValidated(packet.shared_from_this());