security: Fixing compilation errors and providing backward compatible interface for Signature*

This commit includes missed includes and "re-introduction" of
setKeyLocator method into Signature interface.  This method was used in
dependent applications as part of SignatureSha256With(Rsa|Ecdsa) data
structures.

Change-Id: I788f687600bb2729ea8265e23d4a31b990b8ed96
Refs: #1750, #1755
diff --git a/src/signature-info.cpp b/src/signature-info.cpp
index 1b8203d..2543dd2 100644
--- a/src/signature-info.cpp
+++ b/src/signature-info.cpp
@@ -20,6 +20,7 @@
  */
 
 #include "signature-info.hpp"
+#include <boost/lexical_cast.hpp>
 
 namespace ndn {