builder: Don't warn for unknown properties starting with x-
builder: Don't warn for unknown properties starting with x-

Fixes https://github.com/flatpak/flatpak/issues/809

handsome-feng/flatpak,matthiasclasen/flatpak,GeorgesStavracas/flatpak,flatpak/flatpak,GeorgesStavracas/flatpak,flatpak/flatpak,GeorgesStavracas/flatpak,matthiasclasen/flatpak,handsome-feng/flatpak,handsome-feng/flatpak,GeorgesStavracas/flatpak,handsome-feng/flatpak,flatpak/flatpak,matthiasclasen/flatpak,matthiasclasen/flatpak,flatpak/flatpak,matthiasclasen/flatpak,handsome-feng/flatpak,GeorgesStavracas/flatpak,flatpak/flatpak