Don't delete HsTimeConfig.h.in during distclean.

sdist preps the tree via distclean before anything else, which caused
HsTimeConfig.h.in under 'time' to be deleted - even though it should be
included in the resulting tarball for ./configure.

The correct target is 'maintainer-clean'.

I'm guessing the nightlies didn't complain because they run ./boot,
forcing regeneration. NixOS's Hydra does not, though.

Thanks to Peter Simons and Andres Löh for pointing this out.

Signed-off-by: Austin Seipp <aseipp@pobox.com>