Make sure `HAVE_{SEEK,TELL}DIR` are in scope

This is a follow-up to b8fb0a0a9 which made the module exports of
`tellDirStream` and `seekDirStream` CPP-conditional but didn't
explicitly include `HsUnixConfig.h` which defines the respective CPP
defines.

Signed-off-by: Herbert Valerio Riedel <hvr@gnu.org>