[Vc] Work around missing #include <algorithm> in Vc.

Vc/common/algorithm.hh uses std::for_each_n() when it detects C++17, but fails to include <algorithm>. Work around this by including <algorithm> ourselves whenever we include <Vc/Vc>.

Closes: #88 (closed).

enabled an automatic merge when the pipeline for c000d31e succeeds

mentioned in issue #88 (closed)

closed

The problem seems to be that g++-7.2 simply has no std::for_each_n(), see https://godbolt.org/g/BecxqM. So closing this one.