security: Rename security/v2/* to security/*

Security v1-v2 transition has been completed for a while and path
separation is no longer needed. However, this commit preserves
`ndn::security::v2` namespace, but makes it inline.

Some commonly used v2 headers are preserved, but are deprecated and
should be avoided in the dependent software.

Change-Id: I26056cb3d34bcdc0a16e06dea8c1a78c03da2d40
diff --git a/tests/identity-management-fixture.cpp b/tests/identity-management-fixture.cpp
index 3877a23..0c95567 100644
--- a/tests/identity-management-fixture.cpp
+++ b/tests/identity-management-fixture.cpp
@@ -21,7 +21,7 @@
 
 #include "tests/identity-management-fixture.hpp"
 
-#include "ndn-cxx/security/v2/additional-description.hpp"
+#include "ndn-cxx/security/additional-description.hpp"
 #include "ndn-cxx/util/io.hpp"
 
 #include <boost/filesystem.hpp>