If flatpak is built from a separate build directory, code generation
fails because the directory structure is not in place. Create the
necessary directories before code generation.
We got an error in gnome-continuous:
../permission-store/permission-store.c:27:35: fatal error: permission-store-dbus.h: No such file or directory
#include "permission-store-dbus.h"
Not sure how this happens, but I hope this will fix it.