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>