MDEV-26195 fixup: Format mismatch in mariabackup

write_backup_config_file(): Use the correct format for innodb_undo_tablespaces. The data type was changed in commit ca501ff.

Author: dr-m

extra/mariabackup/backup_mysql.cc

@@ -1608,7 +1608,7 @@ bool write_backup_config_file()
 "innodb_log_file_size=%llu\n"
 "innodb_page_size=%lu\n"
 "innodb_undo_directory=%s\n"
-"innodb_undo_tablespaces=%lu\n"
+"innodb_undo_tablespaces=%u\n"
 "innodb_compression_level=%u\n"
 "%s%s\n"
 "%s\n",