**breaking change**: Switch Face and related classes to v2::KeyChain

security::v2::KeyChain is now exposed as ndn::KeyChain, which should
ensure that dependent code can be mostly compiled.  However, expect code
that explicitly uses the old KeyChain interface to be broken.

Change-Id: I7330d0250d92f3f0f2570ab6d0214ab3dfdd18cc
Refs: #3098
diff --git a/src/security/v2/key-chain.hpp b/src/security/v2/key-chain.hpp
index f0299c3..e564326 100644
--- a/src/security/v2/key-chain.hpp
+++ b/src/security/v2/key-chain.hpp
@@ -503,6 +503,9 @@
 
 } // namespace v2
 } // namespace security
+
+using security::v2::KeyChain;
+
 } // namespace ndn
 
 #endif // NDN_SECURITY_V2_KEY_CHAIN_HPP