Fix compilation on mac

test/core/network_benchmarks/low_level_ping_pong.c

@@ -238,6 +238,7 @@ static int set_socket_nonblocking(thread_args *args) {
 
 static int do_nothing(thread_args *args) { return 0; }
 
+#ifdef __linux__
 /* Special case for epoll, where we need to create the fd ahead of time. */
 static int epoll_setup(thread_args *args) {
   int epoll_fd;
@@ -258,6 +259,7 @@ static int epoll_setup(thread_args *args) {
   }
   return 0;
 }
+#endif
 
 static void server_thread(thread_args *args) {
   char *buf = malloc(args->msg_size);