fix _mcl_hardness values for crops and seeds

crops/rhubarb.lua

@@ -46,6 +46,7 @@ local def = {
 		handy = 1, snappy = 3, flammable = 2, plant = 1, attached_node = 1,
 		not_in_creative_inventory = 1, growing = 1
 	},
+	_mcl_hardness = farming.mcl_hardness,
 	is_ground_content = false,
 	sounds = farming.sounds.node_sound_leaves_defaults(),
 	minlight = 10,