[!603] Fix compilation of varianttest when compiling with libc++

Merge branch 'bugfix/fix-varianttest-with-libc++' into 'master'

ref:core/dune-common When using clang with libc++, the vector header does not
get pulled in implicitly by some part of the standard library, causing
compilation of varianttest to fail.

See merge request [!603]

  [!603]: gitlab.dune-project.org/core/dune-common/merge_requests/603
7 jobs for master in 6 minutes and 49 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Test
passed #76706
duneci
debian:10 clang-7-libcpp-17

00:05:05

passed #76711
duneci iset:avx allowed to fail
debian:10 gcc-7-14--expensive

00:06:08

passed #76705
duneci
debian:10 gcc-8-noassert-17

00:06:49

passed #76708
duneci
debian:9 clang-3.8-14

00:05:27

passed #76707
duneci
debian:9 gcc-6-14

00:06:06

passed #76709
duneci
ubuntu:16.04 gcc-5-14

00:04:02

passed #76710
duneci
ubuntu:18.04 clang-6-17

00:03:59