Add dependency on musl standalone implementation of "fts.h" in case of
pkgsMusl/pkgsStatic build; this interface is provided as part of libc
proper by glibc.
Upstream archive has no version and is not reproducible. I doubt version 1.05
would ever appear, but still.
Suggested-By: @SuperSandro2000
Co-authored-by: Dmitry Bogatov <git#v1@kaction.cc>