agent.c: Add missing docu to libssh_auth group

Signed-off-by: Norbert Pocs <npocs@redhat.com>
Reviewed-by: Jakub Jelen <jjelen@redhat.com>
(cherry picked from commit f8ba12f0a6)

src/agent.c

@@ -150,6 +150,12 @@ static void agent_set_channel(struct ssh_agent_struct *agent, ssh_channel channe
   agent->channel = channel;
 }
 
+/**
+ * @addtogroup libssh_auth
+ *
+ * @{
+ */
+
 /** @brief sets the SSH agent channel.
  * The SSH agent channel will be used to authenticate this client using
  * an agent through a channel, from another session. The most likely use
@@ -189,6 +195,10 @@ int ssh_set_agent_socket(ssh_session session, socket_t fd){
   return SSH_OK;
 }
 
+/**
+ * @}
+ */
+
 void ssh_agent_close(struct ssh_agent_struct *agent) {
   if (agent == NULL) {
     return;