kamailio-mysql-modules: support percona-server-client-5.1 as alternative to mysql-client dependency

Currently ngcp-services-ce fails to install on Debian/squeeze
for release-trunk-squeeze because of:

| # apt-get install ngcp-services-ce
| [...]
| The following packages have unmet dependencies:
|  ngcp-services-ce : Depends: kamailio-mysql-modules but it is not going to be installed
|                     Depends: mysql-client or
|                              percona-server-client-5.1 but it is not going to be installed
|                     Depends: mysql-server or
|                              percona-server-server-5.1 but it is not going to be installed

I might be totally wrong but I don't see any reason why it should
fail besides apt not being able to fully resolve the percona
packages as valid mysql-client package:

| # apt-cache show percona-server-client-5.1  | grep Provides
| Provides: mysql-client

since:

| # apt-get install ngcp-services-ce percona-server-client-5.1 percona-server-server-5.1

just works™. :-/

So at least give it a try before we dig deeper into apt's
resolver...