build-aux: Disable DeckLink in the Flatpak

[tytan652]

Description

Disable a feature that might never be available in Flatpak since BlackMagic does not provide a stable link to download it as extra-data.

Motivation and Context

Since the Flatpak became part of the repo, nothing has changed on BlackMagic side.

DeckLink libraries are still not redistributable, which makes enabling DeckLink feature in the Flatpak impossible without involving customization on the end-user side which is not how Flatpak is designed for.

How Has This Been Tested?

It builds

Types of changes

• Flatpak cleanup

Checklist:

• [x] My code has been run through clang-format.
• [x] I have read the contributing document.
• [x] My code is not on the master branch.
• [x] The code has been tested.
• [x] All commit messages are properly formatted and commits squashed where appropriate.
• [x] I have included updates to all appropriate documentation.