build-aux: Disable DeckLink in the Flatpak
Since the Flatpak became part of the repo, nothing has changed on
BlackMagic side.
DeckLink libraries are still not redistributable and without stable
download link so even extra-data is not usable.
This makes enabling DeckLink feature in the Flatpak impossible without
involving customization on the end-user side which is not how Flatpak is
designed for.
tytan652