Fix default registry of Haiku font backend

Fix default registry of Haiku font backend

* src/haiku_support.h (struct haiku_zoom_event): Fix coding
style.
(enum haiku_font_specification): Move FSPECs over here.
* src/haikufont.c (haikufont_apply_registry)
(haikufont_get_fallback_entity, haikufont_pattern_to_entity)
(haikufont_spec_or_entity_to_pattern, haikufont_list): Use
`iso10646-1' as the default registry instead of `utf8', which is
not a registry.