cmake: Fix version generation for included header file

[PatTheMav]

Description

Fix generation of version header when CMake 3.0 variant is used.

Motivation and Context

The generated version header file relies on project version variables which CMake only generates when an actual project exists (which was removed in the update as only the main project obs-studio exists).

How Has This Been Tested?

Tested compilation on Windows 11 with updated CMake file.

Types of changes

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

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.