set is_ground_content for all (thx SwissAlpS)

crops/lettuce.lua

@@ -25,6 +25,7 @@ local def = {
 		handy = 1, snappy = 3, flammable = 2, plant = 1, attached_node = 1,
 		not_in_creative_inventory = 1, growing = 1
 	},
+	is_ground_content = false,
 	sounds = farming.sounds.node_sound_leaves_defaults()
 }