common: Add missing check for USE_SYSTEM_HELPER
common: Add missing check for USE_SYSTEM_HELPER

If building with --disable-system-helper, common/flatpak-dir.c might
still try to use polkit APIs. A check for libmalcontent was already in
place but not enough.

flatpak/flatpak,matthiasclasen/flatpak,matthiasclasen/flatpak,flatpak/flatpak,flatpak/flatpak,flatpak/flatpak,matthiasclasen/flatpak,flatpak/flatpak,matthiasclasen/flatpak,matthiasclasen/flatpak