libobs: Make video frame header public

[cg2121]

Description

This fixes compilation of third party plugins that need this header.

Motivation and Context

My plugin, https://github.com/cg2121/obs-decklink-output-filter, currently fails compilation on macOS, as the header cannot be found.

How Has This Been Tested?

Hasn't been tested because the plugin template pulls the released OBS Studio. It doesn't fail on my Linux machine, just on macOS in the CI. I can't test on macOS because I don't have an up to date machine. It should just be a harmless change, though.

Types of changes

• Bug fix (non-breaking change which fixes an issue)

Checklist:

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