util: use ndn::io::IoEncoding::BASE64 instead of BASE_64

refs #3741

Change-Id: I507756249daa0c67b2f133243f3b0e7c39f6812a
diff --git a/tools/ndns-add-rr-from-file.cpp b/tools/ndns-add-rr-from-file.cpp
index a08d26a..5394b19 100644
--- a/tools/ndns-add-rr-from-file.cpp
+++ b/tools/ndns-add-rr-from-file.cpp
@@ -121,7 +121,7 @@
       ioEncoding = ndn::io::HEX;
     }
     else if (encoding == "base64") {
-      ioEncoding = ndn::io::BASE_64;
+      ioEncoding = ndn::io::BASE64;
     }
     else {
       std::cerr << "Error: not supported encoding format '" << encoding