include: Mark ssh_log as depcrecated.

include/libssh/libssh.h

@@ -425,7 +425,7 @@ LIBSSH_API void *ssh_get_log_userdata(void);
 LIBSSH_API int ssh_set_log_userdata(void *data);
 
 /* legacy */
-LIBSSH_API void ssh_log(ssh_session session,
+SSH_DEPRECATED LIBSSH_API void ssh_log(ssh_session session,
                                        int prioriry,
                                        const char *format, ...) PRINTF_ATTRIBUTE(3, 4);