Use https for git submodule
Please use https for git submodule instead of git. This will allow me to package it as a flatpak correctly.
meson.build:38:0: ERROR: Missing Meson file in /home/bilelmoussaoui/Projects/passbook/subprojects/po/meson.build
A full log can be found at /home/bilelmoussaoui/.var/app/org.gnome.Builder/cache/gnome-builder/projects/passbook/builds/default-host-master/meson-logs/meson-log.txt
Fails to clone because of the git repo.