Mgv6: Add stairs to desert stone dungeons

As with the other mapgens, now that wide stairs in dungeons are
possible we can now finally add stairs to desert stone dungeons.
Re-order some lines.

src/mapgen_v6.h

@@ -123,6 +123,7 @@ public:
 	content_t c_cobble;
 	content_t c_mossycobble;
 	content_t c_stair_cobble;
+	content_t c_stair_desert_stone;
 
 	MapgenV6(int mapgenid, MapgenV6Params *params, EmergeManager *emerge);
 	~MapgenV6();