KWIVER consumers can't build against an install. Variant.hpp publicly exposed

[dstoup]

Back in November a PR landed that has variant.hpp included in the header, visit.h. Visit.h is installed but variant.hpp is not which prevents any external build from succeeding against an installed KWIVER. We either need to remove variant.hpp from 'internal' or fix visit so variant.hpp is kept internal.

[daniel-riehm]

I would opt towards making it public - it's a third-party implementation of C++17 standard library file <variant>, and we wouldn't feel strong reasons to not include <variant> in our publicly-included files.

(Of course, the real solution here is to upgrade to C++17.)

[daniel-riehm]

Tentatively closing as issue seems to have been resolved by #1522