Compile all sql plugins in CI

Currently not all sql plugins are compiled within the CI due to missing dependencies. This can lead to code checked in which is not compilable by accident (already happend here.
Therefore we need at least one CI system where this code gets tested before it gets merged to git.
For Interbase/Firebird this was done recently: https://codereview.qt-project.org/c/qt/qt5/+/577095
For Oracle the provisioning is here, waits to get merged: https://codereview.qt-project.org/c/qt/qt5/+/585590
The only one left then is IBM DB2. Sadly they don't provide a package without creating an account.