[Univalue] add univalue over subtree

similar to secp256k1 include and compile univalue over a subtree

src/test/rpc_wallet_tests.cpp

@@ -31,7 +31,7 @@
 #include <boost/format.hpp>
 #include <boost/filesystem.hpp>
 
-#include "univalue/univalue.h"
+#include <univalue.h>
 
 using namespace std;