qt submodule needs updating

I know that according to https://github.com/wkhtmltopdf/wkhtmltopdf/issues/5160 this project is no longer maintained however this is an effortless fix.

Currently the qt submodule is on commit cb88757 which doesn't include subsequent commits that fixed issues with gcc 9+.

So when you do:

git clone --recursive https://github.com/wkhtmltopdf/wkhtmltopdf.git

you get a version that doesn't compile with gcc 9+, until you do:

git submodule update --recursive --remote

I discovered this using wkhtmltopdf/packaging to build a package for Fedora 37 which I eventually managed to do for aarch64 and x86_64. I will open a PR with my work in due course.