mpv: Disable Swift as a workaround for PR pkg/54426

According upstream pull request 6612
(<https://github.com/mpv-player/mpv/pull/6612/>) and as noticed
via PR pkg/54426 newer Xcode (10.2) dropped support for Swift 3
that is used in mpv-0.29.1.

Workaround that by disabling swift support.

This will probably not needed and should be removed in next stable
mpv version.

(leot)