Botan: Don't unconditionally build in release mode.

Mixing debug and release mode is a bad idea in general and does
not even work on some platforms.

Change-Id: I0e112de1f3e878c5d8f3b57789df194c772848ae
Reviewed-by: Andreas Holzammer <andreas.holzammer@kdab.com>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@nokia.com>