artifacts-tar: when including `.dll` files, don't forget the unit-tests

artifacts-tar: when including `.dll` files, don't forget the unit-tests

As of recent, Git also builds executables in `t/unit-tests/`. For
technical reasons, when building with CMake and Visual C, the
dependencies (".dll files") need to be copied there, too, otherwise
running the executable will fail "due to missing dependencies".

The CMake definition already contains the directives to copy those
`.dll` files, but we also need to adjust the `artifacts-tar` rule in
the `Makefile` accordingly to let the `vs-test` job in the CI runs
pass successfully.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>