modernize cpp echo server

src/emper-echo-server.cpp

@@ -28,6 +28,7 @@ void client_func(void* arg) {
 	int client_fd = *(int*)arg;
 	free(arg);
 
+	// NOLINTNEXTLINE(modernize-avoid-c-arrays)
 	char buf[BUF_MAX];
 	for (;;) {
 #ifdef USE_ASYNC_IO