UI: Change how the status bar gets weak stream output

[cg2121]

Description

The changes obs_output_get_weak_output to OBSGetWeakRef to be consistent with the other UI code.

Motivation and Context

Consistent code

How Has This Been Tested?

Ran stream and nothing broke

Types of changes

• Code cleanup (non-breaking change which makes code smaller or more readable)

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.