Change logging level from ERROR to INFO for no DNS64 prefix

Actually, it is not an error because the network environment
may not have a NAT64 server.

Test: atest
Change-Id: Id7ad35914ac9736c9c465dce9f7b21fdffe76fa7
diff --git a/ResolverController.cpp b/ResolverController.cpp
index c59676a..22878f3 100644
--- a/ResolverController.cpp
+++ b/ResolverController.cpp
@@ -281,7 +281,7 @@
 int ResolverController::getPrefix64(unsigned netId, netdutils::IPPrefix* prefix) {
     netdutils::IPPrefix p = mDns64Configuration.getPrefix64(netId);
     if (p.family() != AF_INET6 || p.length() == 0) {
-        LOG(ERROR) << "No valid NAT64 prefix (" << netId << ", " << p.toString().c_str() << ")";
+        LOG(INFO) << "No valid NAT64 prefix (" << netId << ", " << p.toString().c_str() << ")";
 
         return -ENOENT;
     }