cmake: Reintroduce macOS font size hack

[gxalpha]

Description

eea0e3d (part of #9653) removed the equivalent of this as part of the theming overhaul but didn't provide a replacement (or other reasoning), resulting in text being tiny again. Let's add this back, at least until a proper solution is found.

Motivation and Context

Don't want tiny text.
There has been chatter about potentially adding OS-specific variables or something to the theming system. This however hasn't happened and since we're already in the deps updating phase it's likely it isn't happening. I don't want to have a beta ship with the tiny text so lets just re-add the hack.

How Has This Been Tested?

macOS 14.4.1

See the screenshot of this PR vs current master vs OBS 30.1 (for reference).

This PR: Current master branch: 30.1:

Also went through the program (in particular: Settings, About Dialog, Source Properties, Advanced Audio Properties) to confirm nothing else exploded.

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.