obs-ffmpeg: Fix build when ENABLE_HEVC=false

[stephematician]

Description

Simple fix for missing pre-processing condition around HEVC-specific code.

Motivation and Context

Build with ENABLE_HEVC: false fails due to missing definition(s) + implicit declaration.

How Has This Been Tested?

Make with -DENABLE_HEVC=OFF or use CMakeUserPresets.json to define custom build without HEVC.

Types of changes

• Tweak (non-breaking change to improve existing functionality)

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.