Fix compiling mydns module without gmysql module

This fixes the last case in #544:
https://github.com/PowerDNS/pdns/issues/544#issuecomment-32457187

It's still a dirty hack, and we should pull the parts that mydns
and gmysql have in comming into a common library.
Merge pull request #1796 from rubenk/fix-compiling-mydns-as-module

Fix compiling mydns module without gmysql module