[map] Fix bots

[map] Fix bots

Older compilers don't like calling iter() from return-type decltype()

../src/hb-map.hh:226:12: error: cannot call member function 'decltype ((((+ hb_array(((const hb_hashmap_t<K, V, kINVALID, vINVALID>*)this)->hb_hashmap_t<K, V, kINVALID, vINVALID>::items, (((const hb_hashmap_t<K, V, kINVALID, vINVALID>*)this)->hb_hashmap_t<K, V, kINVALID, vINVALID>::mask ? (((const hb_hashmap_t<K, V, kINVALID, vINVALID>*)this)->hb_hashmap_t<K, V, kINVALID, vINVALID>::mask + 1) : 0))) | hb_filter((& hb_hashmap_t<K, V, kINVALID, vINVALID>::item_t:: is_real))) | hb_map((& hb_hashmap_t<K, V, kINVALID, vINVALID>::item_t:: get_pair)))) hb_hashmap_t<K, V, kINVALID, vINVALID>::iter() const [with K = const hb_serialize_context_t::object_t*; V = unsigned int; K kINVALID = 0u; V vINVALID = 0u; decltype ((((+ hb_array(((const hb_hashmap_t<K, V, kINVALID, vINVALID>*)this)->hb_hashmap_t<K, V, kINVALID, vINVALID>::items, (((const hb_hashmap_t<K, V, kINVALID, vINVALID>*)this)->hb_hashmap_t<K, V, kINVALID, vINVALID>::mask ? (((const hb_hashmap_t<K, V, kINVALID, vINVALID>*)this)->hb_hashmap_t<K, V, kINVALID, vINVALID>::mask + 1) : 0))) | hb_filter((& hb_hashmap_t<K, V, kINVALID, vINVALID>::item_t:: is_real))) | hb_map((& hb_hashmap_t<K, V, kINVALID, vINVALID>::item_t:: get_pair)))) = hb_map_iter_t<hb_filter_iter_t<hb_array_t<hb_hashmap_t<const hb_serialize_context_t::object_t*, unsigned int, 0u, 0u>::item_t>, bool (hb_hashmap_t<const hb_serialize_context_t::object_t*, unsigned int, 0u, 0u>::item_t::*)() const, const<anonymous struct>&, 0u>, hb_pair_t<const hb_serialize_context_t::object_t*, unsigned int> (hb_hashmap_t<const hb_serialize_context_t::object_t*, unsigned int, 0u, 0u>::item_t::*)() const, 0u>]' without object
     + iter()
            ^
../src/hb-meta.hh:58:41: note: in definition of macro 'HB_AUTO_RETURN'
 #define HB_AUTO_RETURN(E) -> decltype ((E)) { return (E); }
                                         ^