Add ambrosia seed as a somewhat rare loot for trash.

Why is this good:
- Low level illegal stuff to do
- I can make my pothead characters
- This is like the most common illegal drug ever, so having it being harder to get than size guns or basically anything in R&D is weird.
- What are you going to do with weed besides smoke it anyway?

code/game/objects/structures/trash_pile.dm

@@ -221,6 +221,7 @@
 	var/path = pick(prob(6);/obj/item/weapon/storage/pill_bottle/tramadol,
 					prob(4);/obj/item/weapon/storage/pill_bottle/happy,
 					prob(4);/obj/item/weapon/storage/pill_bottle/zoom,
+					prob(4);/obj/item/seeds/ambrosiavulgarisseed,
 					prob(4);/obj/item/weapon/gun/energy/sizegun,
 					prob(3);/obj/item/weapon/material/butterfly,
 					prob(3);/obj/item/weapon/material/butterfly/switchblade,