libobs: Fix canvas private duplicated scenes

[exeldro]

Description

Fix calling obs_scene_duplicate with OBS_SCENE_DUP_PRIVATE_REFS not setting canvas

Motivation and Context

When you use studio mode with duplicate scene you get this warning in the OBS log file:
Attempted to add Scene without specifying a canvas! Using default canvas instead.

How Has This Been Tested?

On windows by enabling studio mode

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.