Update postgresql-libpq.cabal

Fix issue #12
Credit goes to the maintainer of the OpenBSD package, David Coppa.
Tested on OpenBSD 5.4-release
Update postgresql-libpq.cabal

Conditional added for OpenBSD (as in Leon Smith's comment to pull request #13).
Works on OpenBSD 5.4-release and Debian "wheezy".
Merge pull request #13 from AlexanderAA/master

Also worth linking to a discussion of these issues on the OpenBSD mailing list:

http://openbsd.7691.n7.nabble.com/fix-hs-postgresql-libpq-after-kerberos-update-td230192.html