Don't trigger no_fallthrough_only if next case is @unknown

[marcelofabri]

Fixes #2696

[SwiftLintBot]

	12 Messages
📖	Linting Aerial with this PR took 3.19s vs 3.11s on master (2% slower)
📖	Linting Alamofire with this PR took 5.07s vs 5.1s on master (0% faster)
📖	Linting Firefox with this PR took 14.64s vs 13.41s on master (9% slower)
📖	Linting Kickstarter with this PR took 27.37s vs 24.2s on master (13% slower)
📖	Linting Moya with this PR took 2.56s vs 2.59s on master (1% faster)
📖	Linting Nimble with this PR took 2.49s vs 2.25s on master (10% slower)
📖	Linting Quick with this PR took 0.85s vs 0.78s on master (8% slower)
📖	Linting Realm with this PR took 4.18s vs 3.73s on master (12% slower)
📖	Linting SourceKitten with this PR took 1.46s vs 1.5s on master (2% faster)
📖	Linting Sourcery with this PR took 4.85s vs 4.86s on master (0% faster)
📖	Linting Swift with this PR took 37.29s vs 37.41s on master (0% faster)
📖	Linting WordPress with this PR took 30.04s vs 29.9s on master (0% slower)

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