io: clean up compiler warning.

io: clean up compiler warning.

```
/home/rusty/devel/cvs/ccan/ccan/io/test/run-08-read-after-hangup.c: In function ‘main’:
/home/rusty/devel/cvs/ccan/ccan/io/io.h:62:58: warning: declaration of ‘conn’ shadows a previous local [-Wshadow=compatible-local]
   62 |                                          struct io_conn *conn),         \
      |                                          ~~~~~~~~~~~~~~~~^~~~
/home/rusty/devel/cvs/ccan/ccan/typesafe_cb/typesafe_cb.h:35:46: note: in definition of macro ‘typesafe_cb_cast’
   35 |                                              oktype),                   \
      |                                              ^~~~~~
/home/rusty/devel/cvs/ccan/ccan/io/io.h:60:22: note: in expansion of macro ‘typesafe_cb_preargs’
   60 |                      typesafe_cb_preargs(struct io_plan *, void *,      \
      |                      ^~~~~~~~~~~~~~~~~~~
/home/rusty/devel/cvs/ccan/ccan/io/test/run-08-read-after-hangup.c:42:9: note: in expansion of macro ‘io_new_conn’
   42 |         io_new_conn(conn, fds[1], init_writer, conn);
      |         ^~~~~~~~~~~
/home/rusty/devel/cvs/ccan/ccan/io/test/run-08-read-after-hangup.c:36:25: note: shadowed declaration is here
   36 |         struct io_conn *conn;
      |                         ^~~~

```