[subset] minor, use a better type in iteration

In file included from hb-ot-face.cc:34:
In file included from ./hb-ot-kern-table.hh:30:
In file included from ./hb-aat-layout-kerx-table.hh:31:
In file included from ./hb-kern.hh:32:
In file included from ./hb-ot-layout-gpos-table.hh:32:
./hb-ot-layout-gsubgpos.hh:1878:63: error: loop variable '_' binds to a temporary value produced by a range of type 'decltype((hb_forward<hb_filter_iter_factory_t<hb_map_t &, const (anonymous struct at ./hb-algs.hh:331:1) &>>(rhs)(hb_forward<hb_zip_iter_t<hb_iota_iter_t<unsigned int, unsigned int>, hb_array_t<const OT::OffsetTo<OT::RuleSet, OT::IntType<unsigned short, 2>, true>>>>(lhs))))' (aka 'hb_filter_iter_t<hb_zip_iter_t<hb_iota_iter_t<unsigned int, unsigned int>, hb_array_t<const OT::OffsetTo<OT::RuleSet, OT::IntType<unsigned short, 2>, true>>>, hb_map_t &, const (anonymous struct at ./hb-algs.hh:331:1) &>') [-Werror,-Wrange-loop-bind-reference]
    for (const hb_pair_t<unsigned, const OffsetTo<RuleSet>&>& _ : + hb_enumerate (ruleSet)
                                                              ^
./hb-ot-layout-gsubgpos.hh:1878:10: note: use non-reference type 'hb_pair_t<unsigned int, const OffsetTo<OT::RuleSet> &>'
    for (const hb_pair_t<unsigned, const OffsetTo<RuleSet>&>& _ : + hb_enumerate (ruleSet)
         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~