Add Steinberg VST3 SDK

[pkviet]

Description

This adds Steinberg VST3 SDK to obs-deps in the include subfolder of the zips.

Motivation and Context

Allows building of obs-vst3, cf : obsproject/obs-studio#12752

How Has This Been Tested?

Tested on my fork; successfully adds the SDK and allows building of obs-vst3 on windows and macOS.

Types of changes

• New feature (non-breaking change which adds functionality)

Checklist:

• 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.

[RytoEX]

Probably fine as-is. I will investigate sparse-checkout with submodules at a later date.