Add IWYU pragma to base.h

[HazardyKnusperkeks]

So clang-tidy is happy if you include <fmt/format.h> for fmt::formatter.

[vitaut]

Thanks for the PR but I don't think it is correct to mark fmt/base.h as a private header in general. Even you consider just fmt::formatter, fmt/base.h is the header that defines it. You only need to include fmt/format.h or other headers like fmt/std.h if you need specializations defined in those headers.