build: autogen.sh: Use mkdir -p to suppress warning
build: autogen.sh: Use mkdir -p to suppress warning

It's minor but hey, warnings stand out.

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