configure.ac: add docbook-to-man to dbparsers

Debian and Ubuntu uses docbook2x-man, but some other distr like suse
uses docbook-to-man. I think all of them should work on LXC.

Signed-off-by: Qiang Huang <h.huangqiang@huawei.com>
Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>