gitlab-ci: Build decorations in minimal CI run

A lot of integration tests rely on the fact that there is a
window decoration.

The `KWIN_BUILD_DECORATIONS` pretty much only disables
build of Aurorae window decoration plug-in but otherwise
has no impact on KWin's core code base in terms of ifdefs
or dependencies required.

Re-enable building decorations to unbreak those tests.

Signed-off-by: Victoria Fischer <victoria.fischer@mbition.io>