[Ubuntu] Enabled debug packages

[Sebastiaan Couwenberg]

In the recent MapCache thread I wrote:

> It's a shame Launchpad doesn't build debug packages automatically,
> that should be helpful to troubleshoot this further.

It turns out that this can easily be enabled in the PPA details, which I
have subsequently done for ubuntugis-stable, -unstable & -experimental.
And will do so too for -testing after I've recreated it.

Kind Regards,

Bas

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