double-negate test for Stage1Only to fix `make clean`

This was causing `utils/ghctags/dist-install` to not get removed
on `make clean` as `Stage1Only` was unset. So testing for `!= YES` is
less fragile than testing for `== NO` in this case.

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