gossipwith: correctly replace all ccan/io operations in handshake.c.

This is kind of a hack, but let's make it a complete hack.  GCC with
-flto noticed we use different definitions of 'struct io_conn' here
and gave the warning:

ccan/ccan/io/io.h:620:17: warning: type of ‘io_close’ does not match original declaration [-Wlto-type-mismatch]
 struct io_plan *io_close(struct io_conn *conn);
                 ^
ccan/ccan/io/io.c:449:17: note: ‘io_close’ was previously declared here
 struct io_plan *io_close(struct io_conn *conn)
                 ^
ccan/ccan/io/io.c:449:17: note: code may be misoptimized unless -fno-strict-aliasing is used

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

devtools/gossipwith.c

@@ -13,6 +13,7 @@
 
 #define io_write_ simple_write
 #define io_read_ simple_read
+#define io_close simple_close
 
 static struct io_plan *simple_write(struct io_conn *conn,
 				    const void *data, size_t len,
@@ -24,6 +25,11 @@ static struct io_plan *simple_read(struct io_conn *conn,
 				   struct io_plan *(*next)(struct io_conn *, void *),
 				   void *next_arg);
 
+static struct io_plan *simple_close(struct io_conn *conn)
+{
+	return NULL;
+}
+
 #include "../connectd/handshake.c"
 
 /* This makes the handshake prototypes work. */