[Univalue] add univalue over subtree

similar to secp256k1 include and compile univalue over a subtree

src/rpcclient.cpp

@@ -11,7 +11,7 @@
 #include <set>
 #include <stdint.h>
 
-#include "univalue/univalue.h"
+#include <univalue.h>
 
 using namespace std;