VLCKit: fix distribution of libvlc headers on iOS and tvOS

Merged Felix Paul Kühne requested to merge fkuehne/VLCKit:3.0-header-fix into 3.0

This fixes the distribution of the libvlc C API headers, which we previously erroneously distributed for macOS only. As the full libvlc C API is exposed by the VLCKit binary, these headers can be used to directly access it.

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