redwood: add missing flags for decoration placements

thanks to iska

mapgen.lua

@@ -201,7 +201,7 @@ minetest.register_decoration({
     y_min = 1,
     y_max = 100,
     schematic = path .. "redwood_tree.mts",
-    flags = "place_center_y",
+    flags = "place_center_y, place_center_x, place_center_z",
 })
 
 -- banana tree