Update UniValue includes in Zcash-specific code

src/gtest/json_test_vectors.h

@@ -5,7 +5,7 @@
 #include "serialize.h"
 #include "streams.h"
 
-#include "univalue/univalue.h"
+#include <univalue.h>
 
 UniValue
 read_json(const std::string& jsondata);

src/gtest/test_rpc.cpp

@@ -1,5 +1,5 @@
 #include <gtest/gtest.h>
-#include "univalue/univalue.h"
+#include <univalue.h>
 
 #include "chain.h"
 #include "chainparams.h"