Repin luautf8 submodule to a commit reachable from upstream

The previous pin (9b9994d) was a local-only commit made directly on a
detached HEAD in the submodule checkout, never pushed anywhere. Since
.gitmodules still points at the real starwing/luautf8 upstream, any
fresh recursive clone would fail to fetch that submodule ref.

Upstream already carries the identical fix (adding the missing
<limits.h> include) at 1bb70d4, reachable from origin/master and a
descendant of the old pin. Repin there instead.

Author: pathanin