Fix build with protobuf >= 30
 https://protobuf.dev/support/migration/#string_view-return-type

--- a/cvmfs/cache_extern.cc
+++ b/cvmfs/cache_extern.cc
@@ -538,7 +538,7 @@ void *ExternalCacheManager::MainRead(void *data) {
       continue;
     } else {
       PANIC(kLogSyslogErr | kLogDebug, "unexpected message %s",
-            msg->GetTypeName().c_str());
+            msg->GetTypeName().data());
     }
 
     RpcInFlight rpc_inflight;
diff --git a/cvmfs/cache_plugin/channel.cc b/cvmfs/cache_plugin/channel.cc
index 4c52d7604..e74d3aa8a 100644
--- a/cvmfs/cache_plugin/channel.cc
+++ b/cvmfs/cache_plugin/channel.cc
@@ -550,7 +550,7 @@ bool CachePlugin::HandleRequest(int fd_con) {
   } else {
     LogCvmfs(kLogCache, kLogSyslogErr | kLogDebug,
              "unexpected message from client: %s",
-             msg_typed->GetTypeName().c_str());
+             msg_typed->GetTypeName().data());
     return false;
   }