fix area test for platforms in OMT process

[etienneJr]

A cartes.app user noticed that public_transport=platform were never considered as areas. I checked process_openmaptiles.lua and indeed noticed an inconsistency between: local is_highway_area = highway~="" and Find("area")=="yes" and is_closed, only for highway and if highway ~= "" or public_transport == "platform" then, also for public_transport

I modified the first in local is_area = (public_transport == "platform" or Find("area")=="yes") and is_closed since area=yes is not required on public_transport=platform

This modification has been validated in https://codeberg.org/cartes/serveur/pulls/86