Add mapgen_stair_cobble to minetest_game

mods/default/mapgen.lua

@@ -26,6 +26,7 @@ minetest.register_alias("mapgen_stone_with_iron", "default:stone_with_iron")
 minetest.register_alias("mapgen_mese", "default:mese")
 minetest.register_alias("mapgen_desert_sand", "default:desert_sand")
 minetest.register_alias("mapgen_desert_stone", "default:desert_stone")
+minetest.register_alias("mapgen_stair_cobble", "stairs:stair_cobble")
 
 --
 -- Ore generation