Make a function static

Now that connection_dir_about_to_close() is in directory.c, there's
no reason to expose connection_dir_request_failed().

src/or/directory.c

@@ -645,7 +645,7 @@ directory_conn_is_self_reachability_test(dir_connection_t *conn)
  * server due to a network error: Mark the router as down and try again if
  * possible.
  */
-void
+static void
 connection_dir_request_failed(dir_connection_t *conn)
 {
   if (directory_conn_is_self_reachability_test(conn)) {

src/or/directory.h

@@ -49,7 +49,6 @@ int connection_dir_process_inbuf(dir_connection_t *conn);
 int connection_dir_finished_flushing(dir_connection_t *conn);
 int connection_dir_finished_connecting(dir_connection_t *conn);
 void connection_dir_about_to_close(dir_connection_t *dir_conn);
-void connection_dir_request_failed(dir_connection_t *conn);
 void directory_initiate_command(const char *address, const tor_addr_t *addr,
                                 uint16_t or_port, uint16_t dir_port,
                                 int supports_conditional_consensus,