Ship leap-seconds.list

[h-vetinari]

our tzdata contains leapseconds, but not leap-seconds.list, which can apparently be found in some forms of the tzdata distribution (c.f. #26). The issue is that libcxx currently only supports parsing leap-seconds.list, but not leapseconds, so we need to add this to our tzdata to properly support C++20's <chrono>.

@conda-forge/tzdata