Fix format bug

Summary: Fix addition bug instead of string concat

Test Plan: ?? It compiles?  The only call paths are when syscalls fail, it would be a lot of work to stub them out

Reviewed By: mshneer@fb.com

Subscribers: trunkagent, doug, njormrod, folly-diffs@

FB internal diff: D1679713

Tasks: 5604354

Signature: t1:1679713:1415925209:ed05d3d1ce3e7e8db32c4afc86c5f24748d9c65b

folly/io/async/AsyncServerSocket.cpp

@@ -764,7 +764,8 @@ void AsyncServerSocket::dispatchError(const char *msgstr, int errnoValue) {
   while (true) {
     // Short circuit if the callback is in the primary EventBase thread
     if (info->eventBase == nullptr) {
-      std::runtime_error ex(msgstr + errnoValue);
+      std::runtime_error ex(
+        std::string(msgstr) +  folly::to<std::string>(errnoValue));
       info->callback->acceptError(ex);
       return;
     }