data/json/terrain.json: Added FLAMMABLE flag to gas pumps

Ever tried to shoot some holes into gas pumps and light it up so it blows up the zombies while you do an heroic escape even James Bond himself would envy? I have, and I blew myself up because the gas pumps don't catch fire, you have to directly light them up. They lacked the FLAMMABLE flag, this should fix the odd behavior.