win-dshow: Fix CMake preventing Virtual Camera from working

[RytoEX]

Description

The code prior to this change would never add virtualcam.c to the win-dshow target, which resulted in the virtualcam_output not being registered and thus Virtual Camera not working.

Motivation and Context

Want the Virtual Camera to work when correctly configured.

How Has This Been Tested?

Tested locally on Windows 11.

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.