CI: Enable WebRTC on FreeBSD

[emaste]

Description

Install www/libdatachannel and leave ENABLE_WEBRTC at the default of ON.

Motivation and Context

I'm not sure why the Cirrus-CI job previously set -DENABLE_WEBRTC:BOOL=OFF on FreeBSD; installing the libdatachannel package is all that's needed to leave this at the default of ON.

How Has This Been Tested?

Build-tested via Cirrus-CI job: https://cirrus-ci.com/task/5539831604838400

Types of changes

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

Checklist:

• [ ] 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.
• [ ] I have included updates to all appropriate documentation.