pelux.xml: fix meta-qt5 branch
pelux.xml: fix meta-qt5 branch

upstream incorrectly points to the "5.12" branch. repo/git still find
the correct commit, but it looks confusing.

Signed-off-by: Oleksandr Kravchuk <da66fcdb9815331ae441a2bc352cef73cf081e1e@pelagicore.com>

Pelagicore/pelux-manifests,sashko/pelux-manifests