Update makefile

Author: kayla-glick

makefile

@@ -115,8 +115,9 @@ $(OUT_DIR)/sim_worker.js: ui/worker/sim_worker.js
 $(OUT_DIR)/net_worker.js: ui/worker/net_worker.js
 	cp ui/worker/net_worker.js $(OUT_DIR)
 
-$(OUT_DIR)/assets: assets/*
-	cp -r assets $(OUT_DIR)
+$(OUT_DIR)/assets/%: assets/%
+	mkdir -p $(@D)
+	cp $< $@
 	rm -rf $(OUT_DIR)/assets/item_data
 
 binary_dist/dist.go: sim/web/dist.go.tmpl