[gn build] try to fix clang build after 37c030f8

37c030f8 made it so that depending on //libcxx/include
automatically added the copied header dir to the include search path.
For some reason, clang can't build against the copied libcxx headers
(it complains about ldiv_t not being a type). I don't have a mac
to debug right now, but for the clang target this change was
unintentional anyways -- only depend on the copies target, instead
of on the target that also adjusts the include path.