gcc-13 and qt5 build fixes

[sarnold]

I have a fairly current dev environment with:

dev-qt/qtcore-5.15.12-r1
dev-qt/qtxmlpatterns-5.15.12
sys-devel/gcc-13.2.1_p20231216

and with these 2 fixes for Qt version and c++ warn-on-error everything builds fine using a local (tox) build environment and git checkouts for openscap, SSG, and scap-workbench. Thanks!