The ssh_channel_callbacks_struct member name is channel_data_function.

Addressing
client.c: In function ‘show_remote_uptime’:
client.c:107:6: error: ‘struct ssh_channel_callbacks_struct’ has no member named ‘channel_data’
     .channel_data = my_channel_data_function,
      ^~~~~~~~~~~~
client.c:107:21: warning: initialization of ‘long unsigned int’ from ‘int (*)(struct ssh_session_struct *, struct ssh_channel_struct *, void *, uint32_t,  int,  void *)’ {aka ‘int (*)(struct ssh_session_struct *, struct ssh_channel_struct *, void *, unsigned int,  int,  void *)’} makes integer from pointer without a cast [-Wint-conversion]
     .channel_data = my_channel_data_function,
                     ^~~~~~~~~~~~~~~~~~~~~~~~
client.c:107:21: note: (near initialization for ‘cb.size’)

Signed-off-by: Jan Pazdziora <jpazdziora@redhat.com>
Reviewed-by: Andreas Schneider <asn@cryptomilk.org>

include/libssh/callbacks.h

@@ -854,7 +854,7 @@ typedef struct ssh_channel_callbacks_struct *ssh_channel_callbacks;
  * @code
  * struct ssh_channel_callbacks_struct cb = {
  *   .userdata = data,
- *   .channel_data = my_channel_data_function
+ *   .channel_data_function = my_channel_data_function
  * };
  * ssh_callbacks_init(&cb);
  * ssh_set_channel_callbacks(channel, &cb);