add missing ENABLE_MDNS in hyperiond.cpp

[Links2004]

Summary

build with out MDNS does not work:

hyperion.ng-git/src/hyperion.ng/src/hyperiond/hyperiond.cpp:298:9: error: ‘_mDNSProvider’ was not declared in this scope; did you mean ‘RRProvider’?
  298 |         _mDNSProvider->thread()->start();

What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• Code style update
• Refactor
• Docs
• Build-related changes
• Other, please describe:

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Does this PR introduce a breaking change? (check one)

• Yes
• No

If yes, please describe the impact and migration path for existing setups:

The PR fulfills these requirements:

• When resolving a specific issue, it's referenced in the PR's body (e.g. Fixes: #xxx[,#xxx], where "xxx" is the issue number)

If adding a new feature, the PR's description includes:

• A convincing reason for adding this feature
• Related documents have been updated (docs/docs/en)
• Related tests have been updated

PLEASE DON'T FORGET TO ADD YOUR CHANGES TO CHANGELOG.MD

• Yes, CHANGELOG.md is also updated

To avoid wasting your time, it's best to open a feature request issue first and wait for approval before working on it.

Other information:

[Links2004]

for rev, this was found during the AUR package build:
https://aur.archlinux.org/packages/hyperion.ng-git#comment-959243