cmake: Make obs-core-modules header public

[FiniteSingularity]

Description

This PR makes the obs-core-modules.h header public to remain consistent with other headers in the libobs/cmake directory by adding PUBLIC to the target_sources line in obs-core-modules.cmake

Motivation and Context

This brings obs-core-modules.cmake in line with all other library files/headers handled by libobs .cmake files.

How Has This Been Tested?

The cmake build process has been run successfully with these changes.

Types of changes

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

Checklist:

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

[tytan652]

LGTM