fix typo

Change-Id: I95c500fe01b9f4e2acae2c3294a6b64afca746b3

Author: johnlanni

source/common/protobuf/utility.h

@@ -583,7 +583,9 @@ class MessageUtil {
 #if defined(HIGRESS) && defined(ENVOY_ENABLE_FULL_PROTOS)
 class HashCachedMessageUtil : public MessageUtil {
 public:
-  bool operator()(const Protobuf::Message& message) const { return message.GetCachedHashValue(); }
+  std::size_t operator()(const Protobuf::Message& message) const {
+    return message.GetCachedHashValue();
+  }
 
   bool operator()(const Protobuf::Message& lhs, const Protobuf::Message& rhs) const {
     return lhs.GetCachedHashValue() == rhs.GetCachedHashValue();